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President Biden will extend until mid-March a requirement that travelers wear masks on airplanes, trains and buses and at airports and transit stations, a person familiar with the decision said on Wednesday night.

The move to extend the mandate, which was set to expire on Jan. 18, is part of a much broader winter strategy for combating Covid-19 that Mr. Biden is to announce on Thursday, during a visit to the National Institutes of Health. The strategy will also include a new requirement that international travelers be tested for the coronavirus one day before departing for the United States, according to officials at the Centers for Disease Control and Prevention. The mask mandate extension was first reported by Reuters on Wednesday.

Mr. Biden will unveil his strategy as the new variant, known as Omicron, is encircling the globe. On Wednesday, health officials in California announced the first confirmed case of the variant, in a patient in San Francisco who arrived in the United States on Nov. 22 from South Africa.

The president has already imposed travel restrictions on people coming to the United States from eight African nations, including South Africa. Mr. Biden said earlier this week that the restrictions were intended to buy the administration time; his science advisers, including Dr. Anthony S. Fauci, the nation’s leading infectious disease specialist, have made clear that it would only be a matter of time before the variant arrived in the United States.

Much is unknown about the Omicron variant. First identified in Botswana and South Africa, it has prompted concern among scientists and public health officials because of an unusually high number of mutations that have the potential to make the virus more transmissible and less susceptible to existing vaccines.

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California Officials Address First U.S. Omicron Case

Officials confirmed that the first case of the Omicron variant of the coronavirus in the U.S. was reported in San Francisco, and assured the public that the city was prepared to handle the virus.

“Through a sequencing process at U.C.S.F., has identified the first known case, and has formally made public knowledge of that case as it relates to the Omicron variant. This individual, who is a resident of San Francisco, was fully vaccinated, had recently been in South Africa, began her — rather began to travel back into the United States on the 21st, landed on the 22nd. This individual has not been hospitalized. The individuals that this individual has come into contact with have not tested positive yet, to our knowledge, and we are hopeful of full recovery and expect nothing less, based upon what we’ve learned.” “The evidence that an individual with Omicron, identified by sequencing, actually has mild symptoms, is improving, I think is a testimony to the importance of the vaccinations.” “We knew that Omicron was going to be here. We thought it would — it was already here, we just hadn’t detected it yet. So this is cause for concern. But it is also certainly not a cause for us to panic. We are prepared here in the city for this.”

The first United States case of the Omicron variant of the coronavirus was reported in California on Wednesday, prompting Biden administration officials to renew their urgent calls for Americans to get fully vaccinated and, if eligible, a booster shot.

The patient, a traveler who returned to California from South Africa on Nov. 22, is in isolation, and aggressive contact tracing is underway, the Centers for Disease Control and Prevention said in a statement, adding that the individual was fully vaccinated and had mild symptoms that were improving. Close contacts of the individuals had tested negative, the agency said.

The World Health Organization has warned that the risk posed by the variant, a new iteration of the coronavirus first detected in southern Africa, is “very high.” More than 20 countries have found the variant so far.

Public health officials around the world have said for days that they expected the new, mutated form of the virus would quickly find its way to the United States despite its imposition of a travel ban on international travelers from eight southern African nations, a move several other countries have also taken.

But confirmation of the variant’s presence nonetheless was a jolt to President Biden’s efforts to make good on his campaign promise to bring the pandemic to a swift and conclusive end. At the White House on Wednesday, Mr. Biden said that “we’re learning more every single day,” and he vowed that the administration would “fight this variant with science and speed, not chaos and confusion.”

Shortly afterward, Dr. Anthony S. Fauci, the president’s top medical adviser, told reporters that confirmation of the new variant in the United States should persuade unvaccinated Americans to get shots immediately.

“We have 60 million people in this country who are not vaccinated who are eligible to be vaccinated,” Dr. Fauci said. “Let’s get them vaccinated. Let’s get the people vaccinated, boosted. Let’s get the children vaccinated.”

Dr. Fauci expressed optimism that the country would eventually emerge from the grip of the pandemic, saying that “there’s no doubt that this will end.” But he also urged caution, saying there was much that health officials still did not know about the new variant.

Omicron carries more than 50 genetic mutations that in theory may make it both more contagious and less vulnerable to the body’s immune defenses than previous variants. More than 30 of the mutations are in the virus’s spike, a protein on its surface. Vaccines train the body’s immune defenses to target and attack the spike.

Available vaccines may still offer substantial protection against severe illness and death following infection with the variant, and federal officials are calling on vaccinated people to get booster shots. The makers of the two most effective vaccines, Pfizer-BioNTech and Moderna, are preparing to reformulate their shots if necessary, but that will take time.

In California, Gov. Gavin Newsom said the infected person had not been hospitalized. The governor said the individual started feeling mild symptoms on Nov. 25, was tested on Sunday and got a positive result on Monday. Within a day, scientists at the University of California, San Francisco, had determined that it was Omicron.

California health officials said the state was increasing coronavirus testing at airports, focusing on arrivals from countries identified by the C.D.C. as potential sources of the virus. Mr. Newsom said the state would not be intensifying public health restrictions, at least in the short term, but that “we should assume that it’s in other states as well.”

“There is no reason to panic, but we should remain vigilant,” he said in a statement.

The health director of the city of San Francisco, Dr. Grant Colfax, added that “we are still learning about the Omicron variant, but we are not back to square one with this disease.”

In California, some 79 percent of residents have gotten at least one dose of the vaccine, after months of campaigning by state officials. Cases and hospitalizations have largely been trending downward since a Delta variant-driven summer rise.

Following news of the variant’s spread in South Africa, countries around the world curtailed air travel to and from southern Africa, measures that officials there described as unduly punitive, especially in light of the fact that Western countries have failed to deliver sufficient vaccines and logistical support to the continent.

Dutch officials said on Tuesday that they identified cases of the variant a week before Friday, when 13 passengers who arrived on flights from South Africa tested positive for it, signaling that the variant was already present.

The W.H.O. says the emergence of Omicron resulted from vaccine inequity in poor countries. Even so, some nations, including Britain and the United States, have renewed efforts to persuade citizens to get vaccine booster shots as quickly as possible.

As officials scrambled to contain alarm over the detection of a case of the Omicron variant in California, state leaders portrayed the finding as an encouraging — and inevitable — result of the state’s efforts to be prepared.

“This was predictable,” said Gov. Gavin Newsom, speaking at a news conference on Wednesday in the Central Valley, where he encouraged residents to get vaccinated and get booster shots. “And it was not surprising that the state of California detected it.”

State health officials said the discovery of the Omicron variant — in a traveler who returned to California from South Africa on Nov. 22 — would prompt increased Covid-19 testing at California airports, focusing on arrivals from countries identified by the Centers for Disease Control and Prevention as potential sources of the variant.

However, Mr. Newsom — who beat back a recall effort in September that was fueled in part by resistance to the state’s pandemic health restrictions — suggested that for now at least, the state would not tighten public health rules or close schools.

Mr. Newsom said there were “no indications” that such restrictions would be needed “as long as we continue our nation-leading efforts.

State officials had said it would be only a matter of time until the Omicron variant appeared on the West Coast. California is a first U.S. stop or a destination for millions of global travelers, and as recently as Sunday, the state’s Department of Public Health had said that officials were monitoring for signs that the variant had arrived.

Mr. Newsom said the infected patient — a fully vaccinated resident of San Francisco between the ages of 18 and 49 — had been tested after traveling to South Africa, the region where the variant was first detected.

The patient, he said, had landed in California on Nov. 22, developed Covid-19 symptoms three days later and was tested on Nov. 28. The variant was confirmed by scientists at the University of California, San Francisco, the next day.

San Francisco public health officials said the individual was self-isolating, experiencing mild symptoms and assisting with contact tracing. The person had received two doses of the Moderna vaccine but was within the six-month window and had thus not received a booster, Mr. Newsom said.

The governor said the state has partnered with top scientists at the University of California, San Francisco, to sequence Covid-19 cases, built up testing and succeeded in vaccinating many of its residents.

Dr. Mark Ghaly, the state’s secretary of health and human services, went so far as to say that Californians were “proud” to have identified the Omicron case.

Almost 80 percent of California residents have gotten at least one dose of the vaccine, after months of campaigning by state officials. Cases and hospitalizations have been inching mostly downward since a summer rise driven by the Delta variant — though hospitals in areas like the Central Valley, where fewer residents are vaccinated, have filled.

In San Francisco, officials sought to reassure residents.

“San Francisco has one of the highest vaccination rates and lowest death rates in the country because of the actions our residents have taken from the beginning of this pandemic to keep each other safe,” Mayor London N. Breed said in a statement. “We knew that it was only a matter of time until the Omicron variant was detected in our city, and the work that we have done to this point has prepared us to handle this variant.”

In the Bay Area, longstanding mask mandates — some of the state’s most enduring restrictions — have recently been relaxed as the spread of the virus has slowed. Local governments in the Bay Area and in other parts of the state have begun to require businesses to verify vaccination status for entry, and more workers have been required to get their shots — a trend that officials have credited with helping to curtail the transmission of Covid.

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First U.S. Omicron Case Is Detected in California

Dr. Anthony S. Fauci said the first case of the coronavirus variant was reported in the United States, after a vaccinated traveler returned to California from South Africa.

The California and San Francisco Departments of Public Health, and the C.D.C. have confirmed that a recent case of Covid-19 among an individual in California was caused by the Omicron variant. The individual was a traveler who returned from South Africa on November the 22nd and tested positive on November the 29th. The individual is self-quarantining and all close contacts have been contacted, and all close contacts thus far have tested negative. The individual was fully vaccinated and experienced mild symptoms, which are improving at this point. This is the first confirmed case of Covid-19 caused by the Omicron variant detected in the United States. And as all of you know, of course, we’ve been discussing this, we knew that it was just a matter of time before the first case of Omicron would be detected in the United States. And as you know, we know, I’ve been saying it and my colleagues on the medical team, and others have been saying, that we know what we need to do to protect people: get vaccinated. If you’re not already vaccinated, get boosted. If you’ve been vaccinated for more than six months with an mRNA, or two months with J.&J. and all the other things we’ve been talking about — getting your children vaccinated, masking in indoor congregate settings.

As the first case of the Omicron coronavirus variant was reported in the United States, Dr. Anthony S. Fauci, the nation’s top infectious disease expert, cast the detection as a reason for Americans to get vaccinated or boosters and said that public health recommendations had not changed in the presence of the fast-spreading variant.

“We knew that it was just a matter of time before the first case of Omicron would be detected in the United States,” Dr. Fauci said.

The patient, a traveler who returned to California from South Africa on Nov. 22, is in isolation, and aggressive contact tracing is underway, according to the Centers for Disease Control and Prevention. The individual was fully vaccinated and had mild symptoms that are improving. Gov. Gavin Newsom of California said the person had received two doses of the Moderna vaccine but was within the six-month window and had thus not received a booster.

Dr. Fauci said he did not necessarily favor vaccine requirements for domestic air travelers, preferring instead to push to inoculate the remaining unvaccinated people in the country.

He said he was “not so sure” that new testing requirements administration officials are currently weighing for international travelers would have helped catch the case sooner — because the patient took a test immediately after beginning to experience what he described as mild symptoms. And he said it was possible that in the future, the federal government could change its definition of “fully vaccinated” to require international travelers to have received booster shots before entering the United States.

Dr. Fauci stressed the additional protection that booster shots provide across variants of the virus and said Americans should not wait for pharmaceutical companies to develop a booster shoot designed for Omicron.

“Get boosted now,” Dr. Fauci said. “We may not need a variant-specific boost.”

Asked if Americans should feel free to attend holiday parties and drink holiday beverages unmasked, he said it depended on the size of the gathering.

“In a situation with the holiday season, indoor-type settings with family that you know is vaccinated, people that you know, you could feel safe with not wearing a mask and having a dinner, having a reception,” he said. But in larger public settings where it is unclear if everyone is vaccinated, he said, people should wear masks except to eat or drink.

Jill Cowan contributed reporting.

The Japanese government on Thursday reversed its decision a day earlier to require airlines to stop taking new bookings for all flights into the country for the rest of the year.

The move, which had represented a drastic extension of one of the world’s most far-reaching border closures in response to the Omicron variant, prompted worries among Japanese citizens and others who feared being stranded outside Japan.

Japanese airlines said on Thursday that they would resume taking some bookings for flights bound for Japan.

On Monday, Japan barred all nonresident foreigners from entering the country, and on Tuesday it closed its borders to all non-Japanese travelers, including permanent residents of Japan, from 10 southern African countries.

Japan confirmed its second Omicron case, in a traveler from Peru, on Wednesday. Its first case, confirmed on Tuesday, involved a traveler from Namibia.

Japan had only tentatively opened last month to business travelers and students, despite recording the highest vaccination rate among the world’s wealthy democracies — and after seeing its coronavirus caseloads plunge by 99 percent since August.

In South Korea, five cases of the Omicron variant were confirmed on Wednesday among inbound travelers, according to health officials.

On Thursday, as a record 5,266 new daily cases were announced, the Korea Disease Control and Prevention Agency announced that quarantines exemptions would be halted starting Friday for inbound travelers from all countries. Visitors must observe a 10-day quarantine period and three rounds of P.C.R. tests.

The government is also considering expanding genomic sequencing for more inbound travelers, a South Korean Health Ministry spokesman, Son Young-rae, said at a news conference on Wednesday.

Elad Maor initially feared that he might have exposed hundreds of people to the virus when he became the first Israeli to test positive for the new Omicron variant on Saturday morning.

In the three days before his positive results, Dr. Maor, a cardiologist, had attended a large staff meeting at his hospital east of Tel Aviv. He had inserted stents into the arteries of 10 patients. And he had driven to a cardiology conference north of Tel Aviv, sharing the 90-minute car journey with a 70-year-old colleague, and lunched there with five others in a crowded canteen.

Dr. Maor, 45, had attended a piano recital with dozens in the audience, where his 13-year-old played a short piece by Stephen Heller, a Hungarian composer. And finally, last Friday night, Dr. Maor had eaten sea bass at the home of his in-laws, together with his wife and nine other family members.

But of these many people, most of whom had received three shots of the Pfizer-BioNTech vaccine, only his 70-year-old colleague has so far tested positive for the Omicron variant in the five days since.

That number may yet rise, as the virus can take several days to show up in tests, and not every contact has been tested. But at least 50 people have already been screened with a P.C.R. test by Dr. Maor’s hospital, the Sheba Medical Center, and at least 10 of those have been tested at least three times.

These initial results have led the infectious disease experts at Sheba, which houses one of Israel’s leading coronavirus laboratories, to cautiously hope that people who have been vaccinated three times may not be as vulnerable to Omicron as was first feared.

Though Dr. Maor met with many people last week, almost all of them were health care workers or close family members. And the people he had spent the most time with were fully vaccinated and had even recently had a third “booster” shot.

It is important not to extrapolate too much from isolated cases, said Prof. Gili Regev-Yochay, director of the infectious disease epidemiology unit at Sheba, who has helped lead research into the virus. “But this does tell us that, in some cases, Omicron is not as infectious if you’re vaccinated,” Professor Regev-Yochay said. “And I think that’s a major thing.”

To Dr. Maor, who was still in isolation at home on Wednesday night, it was still concerning that he had been hit so hard by the virus, despite being fully vaccinated himself, and despite being a fit nonsmoker without any chronic medical conditions. The cardiologist spent Saturday and Sunday in bed with a fever, sore throat and aching muscles — and only began to feel considerably better on Wednesday afternoon.

“Despite everything, despite the vaccines and the booster, I was in bed for 48 hours,” Dr. Maor said in a phone interview. “If I didn’t have the vaccine, I probably would have ended up in the hospital.”

To Professor Regev-Yochay, the coronavirus expert, her colleague’s experience highlighted the need for travelers to keep testing themselves and avoid busy places for a few extra days after arriving from a country with high infection rates.

Dr. Maor arrived back last Wednesday from London, where he had attended another crowded cardiology conference. Because he had tested negative twice in London, and a third time on arrival back in Israel, he had thought he was safe to operate as normal. But his experience highlighted how the virus may not show up in tests for several days.

That shows that ideally, each new arrival to the country would be tested every morning for at least five days after they land, said Professor Regev-Yochay.

“People should be cautious,” she said. “Every day on a daily basis.”

Travelers reacted with dismay and confusion on Wednesday after the Centers for Disease Control and Prevention said that it plans to toughen coronavirus testing requirements and screening of international fliers bound for the United States because of concern about the Omicron variant.

The agency is considering requiring travelers to provide a negative result from a test taken within 24 hours before departure, among other steps, a spokesman said Tuesday night.

Though the C.D.C. has yet to officially announce any changes, the prospect of them sent travelers searching for updates, booking pre-emptive tests where they could, and scouring airline websites for reservation changes, as the pandemic threatened to upend another December travel season.

“It’s a shame, because travel just opened up again,” said Giritharan Sripathy, who was scheduled to fly to New York from London on Thursday. Mr. Sripathy, who had already taken a P.C.R. test three days before his flight, as required, said he had scheduled a new rapid test for Wednesday as a precaution, “in case they change the rules tomorrow.”

Mr. Sripathy, a Singaporean film producer, said he was concerned that the United States might close off entry to people who are not American citizens or permanent residents. Last year, restrictions like that kept him from returning to the United States, where he works, from a holiday in Singapore for eight months.

“I don’t want a repeat of that,” he said.

The plans to tighten testing requirements reflect growing concern about Omicron, a highly mutated form of the virus that was first documented by researchers in southern Africa and has since been detected in more than a dozen countries around the world, including Britain. Experts say it may be weeks before they will know enough about it to assess how readily it spreads or whether it can evade existing vaccines. In the meantime, countries around the world have imposed travel restrictions, and stock markets have tumbled.

The C.D.C. spokesman, Jason McDonald, said that requiring a negative test within a day of departure, instead of three days, would strengthen the United States’ “already robust protocols” for international travelers, including a requirement that they be fully vaccinated. It was unclear whether the new 24-hour rule would require a particular type of rapid test.

President Biden has said he would announce plans on Thursday for stepping up the fight against the pandemic. It was not clear whether his announcement would include the tighter testing requirements for international travelers, which were first reported by The Washington Post. Mr. McDonald offered no timeline for the C.D.C.’s action.

Mr. McDonald said the C.D.C. continues to recommend that all travelers get a coronavirus test three to five days after arrival in the United States. Unvaccinated travelers should self-isolate and quarantine for seven days after arrival, even if they test negative, the agency says.

Natalie Quillian, the deputy coordinator for the Covid response at the White House, said in an interview on Monday that the administration was “assessing all of our tests to make sure they’re effective in picking up” the Omicron variant, and would remove from the accepted list any tests that were not.

Some travelers said a 24-hour testing rule could make visiting the United States difficult. Paula Tolton, 23, a student in Taipei, Taiwan, who plans to visit relatives in Florida in January, said that she found even the current 72-hour rule nerve-racking because of delays in test processing.

“I’ve had that stress before, when a P.C.R. test didn’t come back when I was supposed to fly here in April,” she said. “I was freaking out.”

Carlos Valencia, a dual Spanish and American citizen whose Seville-based company runs a study-abroad program for American students, said he would put a planned January trip to return to the United States on hold until “there is at least some clarity about whether the new rules make a trip feasible.”

He said shifting rules had driven students in his program “completely crazy” and hampered his business, especially “when you know that variants are going to keep coming.”

Marcus Lamb, a Christian broadcaster in Dallas who had been dismissive of Covid-19 vaccinations, died on Tuesday after contracting the coronavirus, his family said.

Mr. Lamb, 64, who frequently suggested on his Christian television network that people pray instead of getting inoculated, had to be rushed to the hospital when his oxygen levels dropped “all of a sudden” after he contracted the virus, his wife, Joni Lamb, said on the network on Nov. 18.

Mr. Lamb’s wife and son confirmed his death on Tuesday on “Ministry Now,” the show the couple had hosted together.

It was not immediately clear when Mr. Lamb had tested positive for the virus or where he had died. Daystar Television Network, the network the Lambs founded in 1997, did not respond to requests for comment on Wednesday.

As the network’s chief executive, Mr. Lamb hosted his own broadcast as well as the show with his wife. His children regularly appear on the network, which says it reaches more than 108 million households.

Daystar broadcasters, including the Lambs, have repeatedly questioned the efficacy of the vaccine. On May 10, the couple’s show included a segment in which the Covid vaccine was falsely said to be “killing your immune system.”

“We want to warn you, we want to help you, we want to give you an alternative,” Mr. Lamb said.

Instead of getting vaccinated, “we can pray, we can get ivermectin and budesonide and hydroxychloroquine,” he said, referring to several drugs that have not been approved for Covid treatment by the Food and Drug Administration. Mr. Lamb also denounced vaccination mandates for schoolchildren.

On the Nov. 18 show, Ms. Lamb said her husband had contracted the virus after she had. He had diabetes, which exacerbated his symptoms, she said.

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U.N. Asks Donors for $41 Billion in Humanitarian Aid

Martin Griffiths, the United Nations emergency relief coordinator, announced the organization’s 2022 funding drive, citing the need to help 183 million people across the world who are suffering from issues like political instability, climate change and the pandemic.

We estimate for 2022 that there will be 274 million people in need of humanitarian assistance; that’s up by 17 percent from 2021. And to give it a different look, it’s equivalent to the world’s fourth-most populous country. We aim, through this program, to help 183 million – the most vulnerable – again, an increase from last year. And for this, we’re looking for a record $41 billion for the world’s humanitarian needs of the most needy in 2022. The drivers of need are ones which are familiar to all of us. Tragically, it include protracted conflicts; political instability; failing economies, which is something which has been very much to the forefront of our thinking in the last months; the climate crisis, not a new crisis, but one which urges more attention; and of course, the Covid-19 pandemic, of which we have recently been reminded that it has not gone away.

When the United Nations made its last appeal for humanitarian aid funding before the pandemic, it asked donors for about $29 billion. But in the past year alone, there has been a huge jump in the number of people needing help. And so the United Nations is asking for more aid — $41 billion.

As the pandemic enters a third year, and the toll of conflicts and climate change rise, the United Nations said on Thursday that it needed to help 183 million people in 63 countries who are suffering the consequences. That compared with 100 million people at the time of the last appeal, in 2019.

More than 1 percent of the world’s population are now displaced by conflicts and disasters, Martin Griffiths, the United Nations emergency relief coordinator, told reporters in Geneva in announcing the organization’s 2022 funding drive. About 45 million people now face famine, some as a result of climate change.

The pandemic has already forced 20 million people into extreme poverty, he said, citing World Bank estimates, and the new Omicron variant would further ratchet up the economic damage. “With Covid continuing to threaten us and continuing to mutate, we will continue to see increased humanitarian needs,” he said.

The Omicron variant has become by far the most prevalent version of the coronavirus spreading in South Africa, replacing the fading Delta variant, health officials said on Wednesday.

Nearly three-quarters of the 249 positive test samples that were checked genetically in South Africa in November were found to involve the Omicron variant, the National Institute for Communicable Diseases announced.

New cases are rising sharply in South Africa, after having dropped to low levels in recent months. The institute said that there were 8,651 new cases reported on Wednesday, almost twice as many as the day before; as recently as early November, the country was averaging fewer than 300 new cases a day. The share of tests that are coming back positive rose to 16.5 percent, from 10.2 percent on Tuesday, the institute said.

The first Omicron case detected in the United States was announced on Wednesday. The World Health Organization said that at least 23 other countries around the world have reported cases of Omicron, and that it expects that number to grow as scientists around the world sequence more test samples.

In a virtual news conference, W.H.O. experts said that “it is early days” in determining whether the mutations seen in the new variant made it more transmissible or better able to evade vaccine protection, as some experts fear. Dr. Maria Van Kerkhove, the technical lead of the W.H.O.’s coronavirus response, said she expected more information on those issues “within days,” but she emphasized that so far, “there is no evidence to suggest that the vaccines don’t work” against Omicron.

“Vaccines are saving lives,” she said.

Many questions remain about whether this variant causes more severe illness than others, she said, adding that officials in South Africa had seen reports of Omicron cases with symptoms that ranged from “mild disease all the way to severe disease.” To date they had seen no deaths associated with the variant.

Tulio de Oliveira, director of the KwaZulu-Natal Research and Innovation Sequencing Platform, said that the Omicron variant “seems to cause much more breakthrough infections than the previous ones.” He said that he was concerned about the threat of such infections among health workers, which would sap the country’s ability to cope with a surge in cases.

But Mr. de Oliveira cautioned against reading too much into head-to-head comparisons between Omicron and Delta. Omicron might swiftly supplant Delta in countries like South Africa where the older variant was fading, but perhaps not as readily where Delta is still spreading actively, he noted.

The W.H.O. panel emphasized the need for countries to speed up vaccination efforts as much as possible, particularly for vulnerable populations.

The panel also called on health authorities to strengthen sequencing, surveillance and field investigations, including community testing.

The agency strongly advised against imposing “blanket travel bans,” calling instead for “a tailored approach” to travel restrictions that could include quarantine and testing requirements for arrivals.

“Our concern here is that we apply public health principles, not political principles, to select the measures that are used,” said Dr. Michael Ryan, the head of the emergencies program at the W.H.O.

New York City went from being an epicenter of the coronavirus outbreak early in the pandemic to now having a higher percentage of vaccinated residents than the national average. Today, the city is bracing for the arrival of the Omicron variant, which was just detected in California.

“We do anticipate detecting Omicron in New York in the coming days,” the city’s health commissioner, Dr. Dave Chokshi, said on Monday.

Three variants have already been detected in New York City so far this year — Iota, Alpha and Delta. Each new variant provoked worry, but ultimately, proved less devastating than anticipated, thanks, in part, to New York’s relatively high rate of vaccination. About 77 percent of residents have received at least one shot.

Considerable levels of natural immunity from the devastation of the first wave in spring 2020, widespread mask-wearing and robust testing were also factors.

“I think we are potentially more prepared than most,” said Dr. Bernard Camins, an infectious diseases specialist and medical director of infection prevention for the Mount Sinai Health System. “The question is whether at this point people are more fatigued from all those mitigation strategies, and they may not listen.”

New York City’s approach has been to focus on vaccines and boosters.

When asked by a reporter on Monday why he was recommending, rather than mandating, mask-wearing until more is known about Omicron, the mayor said, “What we do not want to do is mix messages about what’s the thing that actually has the most profound impact.”

He added: “The thing that we need to do with urgency is get people vaccinated.”

Federal health officials have directed airlines to provide the Centers for Disease Control and Prevention with the names and contact information of all passengers who boarded flights bound for the United States since Nov. 29 and who had been in southern Africa during the prior two weeks.

The directive, issued Wednesday, applies to passengers who spent time in Botswana, the Kingdoms of Eswatini and Lesotho, Malawi, Mozambique, Namibia and South Africa in the two weeks before flying to the United States. The airlines were directed to provide their names, addresses while in the United States, phone numbers, email addresses, dates of birth and flight information, including seat numbers.

“C.D.C. is issuing this directive to prevent the importation and spread of a communicable disease of public health importance,” a statement from the agency said, an apparent reference to the new Omicron variant of the virus that causes Covid.

Last week, the White House announced a ban on travel from eight countries in southern Africa. And late Tuesday night, the C.D.C. said it planned to toughen virus testing and screening of people flying to the United States by requiring all international passengers to provide a negative result from a test taken within 24 hours of departure.

The new directive was issued under an Oct. 25 order that instructed airlines and aircraft operators to collect specific information from all passengers before boarding, retain the information for 30 days, and transmit it to C.D.C. within 24 hours if requested to do so.

The C.D.C. can share the information with state and local health departments at passengers’ destinations, enabling local health authorities to monitor travelers for Covid, identify symptomatic individuals, notify their contacts, and direct those who are infected to isolate and their contacts to quarantine to avoid further disease spread. They can also use the information to ensure infected individuals get appropriate care.

The order applies to flights that have departed for the United States since Monday morning. Two flights left Johannesburg for the United States that day: a Delta Air Lines flight with more than 300 seats that was headed for Atlanta, and a United Airlines flight with more than 250 seats that was headed for Newark, N.J.

Both flights landed in the United States on Tuesday morning, according to schedules from Cirium, an aviation data provider. Two more United flights are scheduled to leave South Africa for Newark on Wednesday, one from Cape Town and one from Johannesburg.

Delta and United are currently the only two carriers that offer direct or single-layover flights between the countries covered by the C.D.C. order and the United States, according to Cirium schedule data. Delta operates three weekly flights between Johannesburg and Atlanta. United operates five flights a week between Johannesburg and Newark. It also plans to restart seasonal flights between Cape Town and Newark on Wednesday.

Both airlines have said that they are not planning to adjust their flight schedules in response to the administration’s ban on travelers from the region, which took effect on Monday and does not apply to American citizens or lawful permanent residents. The airlines also said that they plan to comply with the C.D.C. order.

Sixty-one people who arrived in the Netherlands on Friday aboard two flights that departed from South Africa tested positive for the virus that causes Covid, including over a dozen who were carrying the new Omicron variant. The number of overall positive cases represented more than 10 percent of the 600 passengers tested.

The World Health Organization’s member states on Wednesday took the first step toward what many governments hope will be a legally binding treaty aimed at strengthening global defenses against pandemics.

A rare special session of the W.H.O.’s governing body agreed to set up an intergovernmental negotiating body that is to meet no later than March to begin negotiating an international agreement intended to ensure a more coherent and equitable response to future pandemics. But the United States and other countries have pushed for a weaker mechanism that would not carry legal obligations for member states.

The W.H.O. director-general, Dr. Tedros Adhanom Ghebreyesus, a strong advocate of a legally binding treaty, hailed the decision as historic, calling it “a once-in-a-generation opportunity to strengthen the global health architecture to protect and promote the well-being of all people.”

The decision marked only the beginning of what promises to be arduous negotiations to try to achieve consensus among the W.H.O.’s 194 member countries. The agreement calls for negotiators to deliver the result of their deliberations in May 2024.

The European Union and Britain have pushed for months for an ambitious treaty or convention that carries legal force. The discovery of the Omicron variant, which has prompted a new wave of travel rules and border closures, primarily targeted at southern African nations where the variant was first identified, has renewed criticism that countries worldwide are acting in a patchwork and discriminatory fashion.

“No better response to the emergence of the Omicron variant than this coming together of the international community behind the effort to strengthen the legal framework underpinning our collective response to pandemics,” Simon Manley, Britain’s ambassador in Geneva, said on Twitter.

The United States described the initiative in a statement as “a momentous step” but, with support from Brazil and other countries, it refused to commit to anything that is legally binding, and kept open the possibility of a weaker instrument.

The international agreement is intended to avoid any repetition of the “fragmented and splintered” steps by nations that Dr. Tedros has said weakened the global response to Covid-19. Proponents of a treaty want commitments to share data, virus samples and technology, and to ensure an equitable distribution of vaccines.

Those issues raise politically sensitive questions of national sovereignty over access to the sites of outbreaks, and potential investigations into origins of diseases — a source of tension between Western governments and China, which has resisted calls for an independent inquiry into the emergence of Covid-19 in the Chinese city of Wuhan in early 2020.

China said this week that it agreed “in principle with the ideas of further strengthening compliance, financing, sharing and information management.” But Beijing appeared wary of a new treaty and cautioned against “politicization, stigmatization and instrumentalization.”

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Travel Limits on Africans ‘Unacceptable,’ U.N.’s Chief Says

António Guterres, the U.N. secretary general, strongly criticized the international response to the Omicron variant and urged governments to consider other measures instead of closing their borders to people from African nations.

With a virus that is truly borderless, travel restrictions that isolate any one country or region are not only deeply unfair and punitive, they are ineffective. We have the instruments to have safe travel. Let’s use those instruments to avoid this kind of — allow me to say travel apartheid, which I think is unacceptable. And we need to continue summoning support and financing for sustainable domestic vaccine manufacturing in Africa. The people of Africa cannot be blamed for the immorally low level of vaccinations available to them, nor should they be collectively punished for identifying and sharing crucial science and health information with the world.

The secretary general of the United Nations, António Guterres, expressed strong criticism of the international response to the Omicron variant, saying that the rush to close borders has especially penalized African nations where the variant was first detected and vaccines were slow to arrive.

“The people of Africa cannot be blamed for the immorally low level of vaccinations available to them,” Mr. Guterres told reporters Wednesday after a meeting with the chairman of the African Union Commission, Moussa Faki Mahamat, at U.N. headquarters in New York. “Nor should they be collectively punished for identifying and sharing crucial science and health information with the world.”

The United States and several European countries were quick to temporarily bar travelers from southern Africa in a defensive move against the spreading of the newly discovered variant. Some countries, including Israel, Japan and Morocco, went further, temporarily barring all foreign travelers.

The borderless nature of the virus, Mr. Guterres said, means that “travel restrictions that isolate any one country or region are not only deeply unfair and punitive — they are ineffective.”

He appealed to “all governments to consider instead repeated testing for travelers, together with other appropriate and truly effective measures.”

“This is the only way to reduce the risk of transmission while allowing for travel and economic engagement,” he said.

Mr. Mahamat, a Chadian statesman who leads the administrative branch of the African Union, joined Mr. Guterres in denouncing the attempts by Western and other non-African countries to effectively isolate Africa because of the Omicron variant, which was first discovered in southern Africa last month.

“As a result of being transparent, much of Africa has been stigmatized,” he said. “This stigmatization cannot be justified.”

Also on Wednesday, the director general of the World Health Organization, Dr. Tedros Adhanom Ghebreyesus, thanked South Africa and Botswana for identifying the new variant and reporting it to global health authorities.

Dr. Tedros said that the two countries should not be discouraged by the “unnecessary” travel bans from other nations in response to the Omicron variant.

The detection of the Omicron variant in southern Africa signals the next stage of the battle against Covid-19: getting many more people inoculated in poorer nations where vaccines have been scarcest in order to deter new mutations from developing.

But while world leaders sometimes talk about this as if it is largely a matter of delivering doses overseas, the experience of South Africa, at least, hints at a far more complex set of challenges.

Like many poor countries, South Africa was made to wait months for vaccines as wealthier countries monopolized them. Many countries still do not have anywhere near enough vaccines to inoculate their populations.

The problems have not ended as shots began arriving in greater numbers.

Neglected and underfunded public health infrastructure has slowed their delivery, especially to rural areas, where storage and staffing problems are common.

And now, there are growing signs in parts of Africa, as well as South Asia, that skepticism or outright hostility toward the Covid vaccines may run deeper than expected.

Deep distrust of governments and medical authorities, especially among rural and marginalized communities, may already be stalling out vaccination drives. The legacy of Western exploitation and medical abuses during and after colonialism is weighing heavily, too.

Misinformation circulating on social media often fills the vacuum, some of it floating in from the United States and Europe, where vaccine refusal has also been an issue.

“There’s no doubt that vaccine hesitancy is a factor in the rollout of vaccines,” said Dr. Matshidiso Moeti, the Africa director of the World Health Organization. News or rumors of potential side effects, she said, “gets picked out and talked about, and some people become afraid.”

Just days before the Omicron variant was first detected, health officials in South Africa turned away shipments of doses from Pfizer-BioNTech and Johnson & Johnson, worried that their stockpile of 16 million shots might spoil amid insufficient demand.

Though only 36 percent of South African adults are fully vaccinated, daily vaccinations have already been flatlining, according to government statistics.

Namibia, Zimbabwe, Mozambique and Malawi have also asked vaccine manufacturers and donors to hold off on sending more shots because they can’t use the supplies they have, according to several health officials involved in the effort to distribute vaccines to developing nations.

President Donald J. Trump tested positive for coronavirus three days before his first debate with Joseph R. Biden Jr. in 2020, two former administration officials said Wednesday.

The White House did not announce the positive test at the time, and the president received a negative result shortly afterward and carried on with a campaign rally and the debate, the officials said. The account was first reported by The Guardian, which cited a forthcoming book by Mr. Trump’s chief of staff, Mark Meadows.

The two former officials, who spoke on condition of anonymity because they were not authorized to discuss the matter publicly, confirmed the timeline on Mr. Trump’s test results contained in “The Chief’s Chief,” by Mr. Meadows, Mr. Trump’s fourth and final White House chief of staff.

The revelation came nearly a year after widespread speculation that Mr. Trump was sick when he first shared a stage with Mr. Biden for their first presidential debate on Sept. 29, months into the pandemic.

The White House declined repeatedly at the time to give a precise chronology of when precisely Mr. Trump first received a positive coronavirus test result. The administration first told the public in the early hours of Oct. 2 that Mr. Trump had tested positive. Mr. Trump was hospitalized later that same day.

GLOBAL ROUNDUP

England’s return to some Covid restrictions on Tuesday provoked a range of emotions: anger, dismay, weariness, and, for some, indifference. From London, festooned with Christmas lights, to Bradford, in northern England, the feeling on the streets was, above all, one of weary resignation.

“I expected it, because the British took so long,” said Ali Hasan, 31, a medical consultant at a hospital in Bradford, referring to the nation’s first lockdown, in March 2020, which he said should have been imposed sooner, were it not for what he described as the “laziness” of the country’s political parties.

The British government ended virtually all restrictions in England over the summer, and has seen a high but relatively stable coronavirus caseload in recent weeks. Even now, it is stopping short of the health pass systems, vaccine mandates and lockdown measures that have been implemented to stem surges elsewhere in Europe.

But it has responded to news of the Omicron variant by severely restricting travel from 10 African countries, reinstating testing and self-isolation requirements for other arrivals, and making face masks compulsory on public transportation and in shops.

Many in England had been anticipating restrictions for a while, and some had started to take matters into their own hands. Though masks are not mandatory in restaurants or cafes, Audrey Mekki, 35, a waitress at Pera, a Mediterranean restaurant in North London, makes a point of wearing one throughout her shift.

“I’m wearing it for my safety, and also for the customers,” she said. “Most don’t mind, but some may not feel comfortable if the waitresses serving their food are not masked.”

Helen Daly, 62, who was in London on a holiday from Cork, Ireland, said she had been surprised by the lax British attitude.

“There’s very little mask-wearing here,” she said. “We were at the theater last night. There were four of us, and we were the only four wearing masks.”

For Matthew Leonard, 22, however, the change on the subway was sudden and visible. He said he arrived in London the night before the new regulations took effect. “It was pretty much how it was before, sort of 50-50,” Mr. Leonard said, “but then this morning I noticed almost all people wearing masks.”

At schools, students age 11 and older are now advised to wear masks in communal spaces. Lucy Long, 41, who has a 10-year-old-daughter, explained that the school drop-off had also changed; parents no longer gather for a conversation at the gates. She supports requiring children to wear masks.

“We don’t know what’s going to happen to these children in 20 years’ time after any infection,” she said.

In Bradford, where Ursula Sutcliffe runs a small plant shop and cafe, she said the successive lockdowns had taken a toll — to say nothing of Brexit, which drove up the cost of her plants, which are imported from the Netherlands.

“We’ve just been in an uphill battle,” Ms. Sutcliffe said. Referring to Britain’s prime minister, she added: “Boris Johnson should never have stopped wearing face masks in the first place. If we’d just kept face masks on, we probably could have just gone about our lives. But he is so eager to say, ‘Yay, we’re back to normal.’ We’re not!”

In other news from around the world:

• Japan asked airlines to stop accepting new bookings for inbound all flights for the rest of the year. However, existing bookings would not be affected. Japan also confirmed its second Omicron case on Wednesday.

• The incoming chancellor of Germany, Olaf Scholz, said he wanted vaccinations in the country to be mandatory, possibly as soon as February. He told a German newspaper that the high rates made the move necessary.

• Ghana and Nigeria are the latest African nations to confirm cases of the variant. In Nigeria, three travelers from South Africa tested positive for the variant, and Ghana detected the variant on Nov. 21.

Among the many unknowns surrounding the new coronavirus variant called Omicron, named after the 15th letter of the Greek alphabet, one has stood out to many English speakers: How is it pronounced?

There is no single, agreed-on English pronunciation, experts say.

One pronunciation, according to Merriam Webster, is “OH-muh-kraan,” with a stress on the first syllable.

A World Health Organization official, Dr. Maria Van Kerkhove, recently said it that way when announcing that the variant was of concern.

In the United States, it is often pronounced “AH-muh-kraan,” Merriam Webster says. Less common are “OH-mee-kraan,” as Prime Minister Boris Johnson of Britain pronounced it this week, or “OH-my-kraan.”

On the New York Times podcast “The Daily,” Apoorva Mandavilli, who reports on the coronavirus and its variants, said she was going with “AH-muh-kraan.”

“I don’t think it really matters that much, honestly,” she said.

The New Oxford English Dictionary gives a pronunciation that differs from those in Merriam-Webster, according to Dr. Andreas Willi, a comparative linguistics professor at Oxford University. “Namely rather like an English phrase ‘o-MIKE-Ron,’” he said.

The word is a compound from the Greek “o mikron,” meaning “small o.” In classical Greek, the word was pronounced with the second syllable sounding like an English “me,” Dr. Willi said.

Peter Sokolowski, editor at large at Merriam Webster, said that because the Greek word is transliterated for pronunciation into English, sounding much as the word “omnipotent” is different from its Latin “omni-potent” origin, then the “AH-muh-kraan” pronunciation “makes perfect sense.”

But, he added, “There isn’t a wrong answer.”

“The question of British versus American pronunciation of the first syllable isn’t really specific to this particular word,” Dr. Willi said. “Compare the British versus American pronunciation of ‘god.’”

The divergences are to do with the name having been adopted as a loanword and used by English speakers in different places at different times, Dr. Willi said.

“When we speak of ‘Paris’ in English, that is also very different from the ‘proper’ French way of pronouncing the same name,” he said. “But it is hardly wrong in a strict sense.”

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First Cases of Omicron Coronavirus Variant Detected in New York

Mayor Bill de Blasio and Gov. Kathy Hochul of New York addressed the several cases of the Omicron variant that have been reported in the United States, including five people in New York City and on Long Island.

“There are five cases identified today in the State of New York. The first one was in Suffolk County. And it was a 67-year-old female who had symptoms, mild symptoms of a headache and a cough. Our second case is a Queens based. The vaccination status is unknown to know whether or not this person have been vaccinated or not. Third case, similar situation. No other further personal details, but the vaccination status is unknown, Queens based. And another case is from Brooklyn, unknown vaccination status. And we just received word of another suspected traveler case here in the city that was identified — in the city in one of the five boroughs. This is not a cause for major alarm. I need to say that, because we do not have enough information. We’re not having shutdowns. We’re not changing our protocols. We are continuing where we are, but making sure that we work in concert together and encourage people to get tested, get tested often get the vaccination. And that, again, is our best defense.” “We have a new situation, but we also know there’s a lot of information we do not yet have about Omicron. We know we now have cases here in New York City. We have to assume that means there’s community spread. We have to assume that means we’re going to see a lot more cases. But we also have tremendous tools, as the governor said. We have a massive vaccination apparatus and that is by far our best capacity to protect everyone against this variant or any other form of Covid.”

New York State has confirmed its first five cases of the Omicron variant, Gov. Kathy Hochul and Mayor Bill de Blasio announced on Thursday evening.

At a rare joint news conference, the leaders encouraged residents to get vaccinated to protect themselves from the coronavirus and urged them not to panic. They said it was too soon to know how much of a threat the variant might pose to the state.

“This is not a cause for major alarm,” Ms. Hochul said, adding that she was not announcing a shutdown or other drastic measures in response to the cases.

Ms. Hochul provided basic details about the five cases: a 67-year-old woman in Suffolk County who returned from South Africa, had mild symptoms and had received at least one dose of a vaccine; two residents in Queens; one resident in Brooklyn and another person in New York City who had traveled recently. The woman in Suffolk County tested positive on Nov. 30.

Ms. Hochul and Mr. de Blasio said that the state was better positioned to fight the new variant than it was early on in the pandemic, when New York was the global epicenter of the health crisis, because so many New Yorkers have gotten vaccinated and officials now have more tools to combat the virus.

“We are in a far better place,” Ms. Hochul said.

Still, Mr. de Blasio said that he expected to see more Omicron cases detected in the coming days.

“We have to assume there’s community spread,” he said.

Earlier in the day, Ms. Hochul had urged attendees of a recent anime convention in Manhattan to get tested because someone who tested positive for the Omicron variant in Minnesota had attended the convention.

The hastily called news conference at City Hall in Lower Manhattan with Ms. Hochul and Mr. de Blasio was eerily reminiscent of the response from officials in early March 2020. When Ms. Hochul’s predecessor, Gov. Andrew M. Cuomo, announced the first coronavirus case in New York on March 1, he said that “this was expected” and “there is no reason for undue anxiety.”

Even before the Omicron variant arrived, coronavirus cases have been rising in New York City — daily case counts have increased more than 75 percent since Nov. 1. While 77 percent of New York City residents have received at least one shot of the vaccine, fewer than 20 percent of adults have gotten their booster shot.

Ms. Hochul and Mr. de Blasio, both Democrats, encouraged people to wear masks indoors, but they have stopped short of a mandate. Ms. Hochul said her greatest concern was overcrowding at hospitals and that officials were working to ensure that hospitals have enough capacity.

“We’re just trying to let people know, ‘We’ve got this’,” she said.

The fact that Ms. Hochul and Mr. de Blasio appeared together was notable. Mr. de Blasio had a notoriously acrid relationship with Mr. Cuomo, and they quarreled frequently over the pandemic response.

Ms. Hochul made clear that she was working closely with Mr. de Blasio.

“This is about unity,” Ms. Hochul said. “There’s a reason we’re here together — to signal that this is a challenge that we’re going to tackle together.”

Several more cases of the Omicron variant of the coronavirus were reported in the United States on Thursday, including five people in New York City and on Long Island, officials said. Other cases were reported in Minnesota, Hawaii, California and Colorado.

Gov. Kathy Hochul of New York said one of the cases in her state was a 67-year-old woman from Suffolk County on Long Island who had recently traveled to South Africa and had tested positive for the new variant, which is highly mutated. The woman had been vaccinated and had mild symptoms of Covid-19, Ms. Hochul said. She said the new cases were “no cause for alarm.”

Minnesota health officials said that a man who lives in the state was infected with the Omicron variant after recently traveling to New York City.

A woman who lives in Colorado became the state’s first case of Omicron, state health officials said, after she recently returned from a trip to southern Africa for tourism. She had been fully vaccinated and was eligible for a booster shot but had not yet received it. She was experiencing mild symptoms and isolating at home.

Health authorities had confirmed on Wednesday the first known U.S. case of the Omicron variant was detected in a San Francisco resident who returned to California from South Africa on Nov. 22. And on Thursday, Los Angeles County confirmed an additional case in California, a fully vaccinated adult who had traveled to South Africa in late November.

But an O‘ahu resident with no history of travel is the first Omicron case in Hawaii. “This is a case of community spread,” the state department of health said in a news release. The individual had previously been infected with the coronavirus but was never vaccinated.

Much remains unknown about Omicron, including whether it is more transmissible and capable of causing more serious illness.

According to the Minnesota Department of Health, the man is a resident of Hennepin County, which includes Minneapolis. He had been vaccinated, and he got a booster shot in early November. He is no longer feeling symptoms, the department said.

The man first developed mild symptoms on Nov. 22, shortly after traveling to New York City for the Anime NYC 2021 convention at the Javits Center, the department said. Officials said the man had not been outside the United States recently. Ms. Hochul said on earlier on Thursday that everyone who attended the convention should get tested for the coronavirus, while Mayor Bill de Blasio said the city had activated its contact tracing program to track down people who attended the event. He added that “we should assume there is community spread of the variant in our city.”

One of his close contacts has since tested positive for the coronavirus, officials said, but because a rapid test was used, scientists have not yet determined whether that person, who is also in Minnesota, had the Omicron variant as well.

Genetic sequencing is required to determine which variant a patient has. In recent months the United States has greatly expanded sequencing, but the process takes time — at the Centers for Disease Control, typically about 10 days — to yield results. Currently, according to the agency’s director, Dr. Rochelle P. Walensky, about 14 percent of all positive P.C.R. tests in the U.S. are being sequenced.

Though the new Omicron variant is grabbing headlines, officials warned that the earlier Delta variant of the coronavirus remains prevalent and is still spreading in the United States, posing a grave risk especially to unvaccinated Americans, who are much more likely than vaccinated people to become severely ill if they are infected.

Gov. Kathy Hochul of New York said on Thursday that everyone who attended a recent anime convention in Manhattan should get tested for the coronavirus, after it was announced that an individual who tested positive for the Omicron variant in Minnesota had attended to the conference.

Five other cases were reported in New York City and on Long Island, the governor announced later in the day. Those included a 67-year-old woman from Suffolk County who had recently traveled to South Africa, was vaccinated and experienced mild symptoms of Covid-19.

Ms. Hochul said the Minnesota resident was also vaccinated and experienced mild symptoms. He had attended the Anime NYC 2021 convention at the Javits Center in Midtown Manhattan.

She urged people who attended the event, which was held from Nov. 19 to Nov. 21, to get tested and said that health officials would be in contact with attendees. The convention hosted 53,000 attendees over three days, according to a spokesman for the Javits Center.

“We do anticipate there’ll be more cases,” Ms. Hochul said. “This is not cause for alarm; it was foreseen ever since it was first reported out of South Africa, that we knew it would come to New York State at some point.”

Much remains unknown about Omicron, including whether it is more transmissible and capable of causing more serious illness. People had to be vaccinated with at least one dose and wear masks to attend the conference. Proof of at least one dose is required for many indoor activities citywide. New York State and New York City do not have universal indoor mask mandates.

Mayor Bill de Blasio said the city had activated its contact tracing program to track down people who attended the event at the Javits Center, adding that “we should assume there is community spread of the variant in our city.”

Ms. Hochul made her remarks on Thursday during a coronavirus briefing in which she introduced Dr. Mary T. Bassett, the newly-appointed New York State health commissioner.

Since Nov. 1, daily case counts in New York City have risen quite a bit, reaching 1,500 newly identified cases a day last week. The rise, driven by the Delta variant, has been steepest in Queens, a borough with vaccination rates well above the citywide average.

“The fact that we have not detected it may mean that it’s still extraordinarily rare here, that it is the proverbial needle in the haystack,” Dr. Bassett said of the Omicron variant. “We now have an exposure and we fully expect that it will be detected in the coming days.”

New York City will require employees at yeshivas, Catholic schools and other private schools to be vaccinated against the coronavirus, as part of the city’s latest push to expand vaccine mandates.

The new directive, which was announced on Thursday, is expected to affect roughly 930 schools and 56,000 employees, city officials said. They will have to show proof they received the first dose of a vaccine by Dec. 20.

“We’re doing everything in our power to protect our students and school staff, and a mandate for nonpublic school employees will help keep our school communities and youngest New Yorkers safe,” Mayor Bill de Blasio said in a statement.

Teachers and other employees at public schools were already required to get vaccinated, and more than 95 percent of the Department of Education’s employees have done so. Students are not required to be vaccinated, and the mayor has resisted setting a mandate for students, as some other American cities have.

As New York City faces growing concerns over the Omicron variant, Mr. de Blasio has encouraged New Yorkers to wear masks indoors, and has put in place a vaccine mandate for child care workers.

His order requiring vaccines for all city employees, including police officers and firefighters, faced resistance from some unions, but most workers eventually got vaccinated. Mr. de Blasio also required proof of vaccination for indoor dining, entertainment and gyms.

The new rules could face opposition from those employed at yeshivas, a group of ultra-Orthodox Jewish private schools, because of resistance to the vaccine.

Ultra-Orthodox neighborhoods, some of which have been ravaged by the coronavirus, have some of the lowest vaccination rates in the city. Misinformation campaigns led to a resistance to restrictions and safety guidelines at the height of the pandemic that at times caused virus cases to surge.

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Biden Outlines Winter Covid Strategy

President Biden laid out his new pandemic strategy, including booster shots for all adults, vaccination sites aimed at families, testing requirements for international travelers and access to free at-home tests.

Today, I’m back to announce our action plan to battle Covid-19 this winter. And it doesn’t include shutdowns or lockdowns, but widespread vaccinations and boosters and testing a lot more. First is expanding the nationwide booster campaign with more outreach, more appointments, more hours, more times and sites to walk in, providing boosters shots for up to 110 million Americans who are eligible for boosters. Launching new family vaccination clinics to make it easier for children, parents and whole families to get vaccinated into one place, and new policies to keep our children in school instead of quarantining them at home. The third piece of this is making free at-home tests more available than ever before, and having them covered by your private insurance plans available in thousands of locations and available community health centers and other sites for the uninsured who don’t have insurance. We’re going to accelerate our efforts to vaccinate the rest of the world and strengthen, and strengthen the international travel rules for people coming to the United States. My plan that I’m announcing today pulls no punches in the fight against Covid-19, and it’s a plan that I think should unite us. We’re going to fight this variant with science and speed, not chaos and confusion.

Over-the-counter rapid coronavirus tests will soon become a much bigger part of the Biden administration’s response to the pandemic and the new Omicron variant.

After a primary focus on vaccination, the White House announced on Thursday that private health insurers would soon have to reimburse patients for such tests. It also said it would make 50 million free tests available for uninsured Americans, to be distributed through health clinics and other sites in rural and underserved communities.

But for consumers who do have insurance, the White House approach will require some legwork. It left some health policy experts questioning why the United States does not purchase tests on behalf of all Americans and provide them at little to no cost, as some European countries have done. Instead, Americans will have to purchase tests and then submit the receipts for reimbursement.

The recent surge in hospitalizations and the arrival of the Omicron variant prompted the Biden administration to try to expand testing availability and improve affordability. The new approach probably rests on expedience, said Andy Slavitt, a former adviser to the White House on coronavirus policy.

“From the standpoint of test makers, this takes away the risk of manufacturing, which should help bring costs down,” he said. “They can quantify the market now.”

If consumers with private coverage are able to navigate the reimbursement process, and use their health plans to pay for Covid tests, that could lead to an unintended consequence: higher prices.

“If the consumer is thinking, ‘I will get reimbursed,’ they won’t really care about the price,” said Ge Bai, a professor of health policy at the Johns Hopkins Bloomberg School of Public Health. “The rule is meant to help promote testing, but it also instills inefficiency into the system.”

António Guterres, the secretary general of the United Nations, has been a vocal critic of the inequity in Covid-19 vaccine distribution. Last month, after the worldwide death toll hit 5 million, he railed at how wealthy countries were “rolling out third doses” while most of the world’s poor remained unvaccinated.

At the annual General Assembly in September, Mr. Guterres described the vaccine disparity as an “obscenity” and said the global community should give itself an F in ethics. He reiterated those views on Wednesday after a meeting with the chairman of the African Union Commission, Moussa Faki Mahamat.

So it was with great reluctance that Mr. Guterres opted to get a booster himself, his spokesman, Stéphane Dujarric, said Thursday, describing his boss as having “agonized over making that decision.”

It is not as though Mr. Guterres is unqualified to receive a third shot. On the contrary, Mr. Dujarric said, at age 72 and with an underlying blood pressure issue, Mr. Guterres is among the category of people that medical experts say should be among the first to get a booster.

With his frequent travels and public appearances, Mr. Dujarric said, the secretary general’s physicians and family — including a daughter who is a doctor — had been pressuring him to accept the extra protection.

And now, with the emergence of the highly mutated Omicron variant, even public-health experts who had opposed booster shots have started to reconsider, reasoning they may offer an effective defense.

Initially vaccinated in New York in January and February with the Moderna vaccine, Mr. Guterres, a Portuguese statesman who is serving his second five-year term as the U.N.’s leader, received the third shot last Friday.

“And I can tell you that this has been a very difficult decision for him,” Mr. Dujarric said at a daily briefing in response to questions about whether Mr. Guterres had received a booster. “There was a lot of back‑and‑forth within his own mind as to whether or not he should get it. I spoke to him at length about it this morning.”

Mr. Guterres also was “fully aware of the symbolism of the secretary general getting a third shot while the global vaccine situation is as horrendous as it is,” Mr. Dujarric said.

Still, he said, given that Mr. Guterres intends to do more traveling, and “given his age and given his function and his responsibilities as secretary general, he decided that getting the booster would be the responsible thing to do.”

The Los Angeles Lakers star LeBron James has been cleared to play in the team’s game against the Los Angeles Clippers on Friday after a false positive coronavirus test landed him in the N.B.A.’s health and safety protocols earlier this week, the league said in a news release.

After initially testing positive on Tuesday, James returned two negative tests, 24 hours apart, administered by league officials. That enabled him to be taken out of the league’s health and safety protocols, according to the league.

In keeping with their enhanced coronavirus protocols after Thanksgiving, N.B.A. officials collected a sample to test on Monday and received a positive result on Tuesday. They used another instrument to test the same sample, and that test returned an inconclusive result. Consequently, James was placed into the league’s health and safety protocols.

James had been with the Lakers in Sacramento, where they were set to play the Kings on Tuesday night. The team arranged for a medical plane to take James back to Los Angeles, according to a person familiar with James’s situation.

James’s teammates were concerned about him.

“Scary situation,” the Lakers’ Anthony Davis said. “He said he’s good. I think he said he was asymptomatic, which is a good sign, but we want to make sure he gets back. Health is most important. It’s bigger than basketball. He has a family.”

Despite testing positive, James never believed he had the coronavirus and consulted a doctor who conducted multiple P.C.R. tests at James’s home that were all negative, according to the person familiar with James’s situation.

James posted a tweet on Wednesday that included five emojis of fish, including one hanging from a fishing hook. Many interpreted the tweet as implying he thought something unusual was happening with his coronavirus testing.

James said before the season that he was vaccinated against Covid-19. Vaccinated players have been tested much less often than unvaccinated players this season. But given the higher chance players might have been exposed to the virus during Thanksgiving gatherings, the N.B.A. increased testing requirements for vaccinated players, according to a memo obtained by The New York Times. All players were required to be tested on Nov. 28, 29 and 30.

The false positive caused James to miss the Lakers’ game Tuesday against the Kings — the 12th game he has missed this season. He has missed 10 games because of injuries and one because of a suspension.

SAN FRANCISCO — For two years, San Franciscans have been pandemic poster children. When Covid-19 hit, the city was among the first in the nation to declare a state of emergency.

Masks have been de rigueur since April 2020. The vaccination rate is among the world’s highest. When the wildly popular In-N-Out Burger stand at Fisherman’s Wharf refused to ask customers for proof of inoculation, the city shut down its indoor dining. “In-N-Out(side),” the city public health department scolded via tweet.

No matter: On Wednesday, as health authorities confirmed that the Omicron variant of the coronavirus had arrived in the United States, the first known case was in San Francisco. The infected person, who authorities said was self-isolating and participating in aggressive contact tracing, had noticed symptoms after returning from South Africa, where the variant was first identified.

Now the city that has led the nation in coronavirus caution is preparing to hunker down. Again. Maybe harder.

“We were thinking of maybe traveling again in the spring,” sighed Linda Wollman, 67, a retiree who has not seen her European relatives since the pandemic started, and who has avoided crowds, restaurants and anyone who is unvaccinated, except her 15-month-old grandson.

“Now I guess we’ll just lay low. Or lay lower. If that’s at all possible.”

Health officials braced for pandemic fatigue across the country this week as word spread that the new variant had reached California, with the inevitability of cases being identified elsewhere.

By Thursday morning, a second case was reported in Minnesota, in a resident who had recently traveled to an anime convention in New York, suggesting that the variant already had begun to circulate.

First identified in Botswana and South Africa, this new iteration of the coronavirus has prompted concern among scientists and public health officials because of an unusually high number of mutations that have the potential to make the virus more transmissible and less susceptible to existing vaccines.

Here is a look at what we know — and don’t — about the variant:

What is Omicron?

The World Health Organization has called Omicron a “variant of concern” and on Monday warned that the global risks posed by it were “very high,” despite what officials described as a multitude of uncertainties. Cases have been identified in 20 countries so far, including Britain, Italy, Belgium and the Netherlands. Although Omicron has not yet been detected in the United States, experts say it is only a matter of time before the variant shows up.

Should we be worried?

Omicron’s discovery has prompted considerable panic across the globe, with a number of countries banning flights from southern Africa, or — like Israel, Japan and Morocco — barring entry of foreign travelers altogether.

But public health experts have urged caution, noting that there is as yet no firm evidence that Omicron is more dangerous than previous variants like Delta, which quickly overtook its predecessors in the United States and other countries.

Much remains unknown about Omicron, including whether it is more transmissible and capable of causing more serious illness. There is some evidence the variant can reinfect people more readily.

There are early signs that Omicron may cause only mild illness. But that observation was based mainly on South Africa’s cases among young people, who are less likely overall to become severely ill from Covid.

Do vaccines protect against Omicron?

Scientists expect to learn much more in the coming weeks. At the moment, they say there is no reason to believe Omicron is impervious to existing vaccines, although they may turn out to be less protective to some unknown degree.

There’s another reason to remain calm: Vaccine makers have expressed confidence they can tweak existing formulations to make the shots more effective against new variants.

Also reassuring: Omicron’s distinctive mutations make it easy to quickly identify with a nasal swab and lab test.

Global Roundup

The Omicron variant could become the dominant version of the coronavirus in Europe in the next few months, the European Center for Disease Prevention and Control said on Thursday, as the continent faced a surge of new infections and deaths that has prompted many governments to tighten restrictions.

The European Union’s infectious disease agency acknowledged in a report that there were “a number of uncertainties” related to the Omicron variant, including how transmissible it is, how likely it is to cause severe symptoms and how effective existing vaccines are against it. But preliminary data already suggests that Omicron, which carries a large number of mutations compared with the original virus, has a “substantial advantage” over previous versions of the virus, the agency said.

Combating the rise of Omicron requires a multilayered approach, including initial vaccinations, booster shots for people over 40, physical distancing and ventilation of indoor spaces, according to Andrea Ammon, the agency’s director. Ms. Ammon cautioned against using blanket travel bans like those recently imposed by many European nations, and said that officials should instead maintain “carefully considered” rules for travelers, including testing, quarantining and sequencing of virus cases, and review those measures regularly.

Speaking to reporters on Wednesday, Ursula von der Leyen, the president of the European Commission, called on the bloc’s member governments to speed up vaccinations and booster shots. As more European countries consider making Covid-19 vaccination mandatory, Ms. von der Leyen said it was “understandable and appropriate” for such mandates to be discussed.

• Greece said on Thursday that it had confirmed a case of the Omicron variant in a man who arrived on the island of Crete from South Africa on Nov. 26. Health Minister Thanos Plevris told reporters that the man was isolating while the authorities carry out contact tracing. Regulations announced on Wednesday to make Covid vaccinations mandatory for residents ages 60 and over, with fines for noncompliance, were approved by parliament on Thursday.

• The Catalonia and Valencia regions of Spain, which together account for more than one-quarter of the country’s population, will require proof of vaccination to enter bars, restaurants gyms and some other venues starting on Friday. The requirement is being challenged in court in those regions, and faces opposition in some other parts of the country as well. The leader of the Madrid region, Isabel Díaz Ayuso, has promised residents that they will not need to show proof of vaccination there.

• Officials in India confirmed that they had found the Omicron variant in the southern state of Karnataka, in two people who tested positive for the coronavirus after overseas air travel. Details of the cases, including the dates when the people were tested and the places they had been, were not immediately disclosed. Officials said that both people experienced mild symptoms and later tested negative.

• An additional seven cases of the Omicron variant have been found in England, Britain’s Health Security Agency confirmed on Thursday, bringing the total number of known cases of the variant in England to 29. A further three cases have also been identified in Scotland, bringing the total there to 13. Those who have tested positive and their contacts are all isolating, the health agency said, and work is being done to determine if they had any links to travel.

At first glance, it looked just like an official New York City public service announcement that listed the top 10 reasons people should get vaccinated against Covid-19.

But a closer reading revealed that the poster at a B43 bus stop in the Crown Heights neighborhood in Brooklyn displayed 10 reasons NOT to get vaccinated. Reason No. 1: “It could kill you.”

The false and blatantly misleading anti-vaccination ad caused an uproar on social media on Thursday at a time when city health officials are pushing hard to boost the vaccination rate as a new threat from the Omicron variant looms. About 69 percent of city residents of all ages have been fully vaccinated.

The poster imitated an official vaccination ad released by the city — it used the same font, layout and even the same shade of robin’s egg blue — but conveyed the opposite message.

It was not immediately clear who was responsible for placing the anti-vaccination ad.

Transit officials said Thursday that the ad had not been approved by the city or sold by the company that installs ads at the bus stops, JCDecaux North America.

Seth Stein, a spokesman for the city’s Department of Transportation, wrote on Twitter that the anti-vaccination ad was being removed immediately.

“Most likely, someone popped open the glass,” he wrote. “Investigation ongoing. Disinformation has no place in our city.”

By 1:15 p.m., a maintenance worker who declined to give his name had replaced the anti-vaccination ad with a city-approved poster that urged people to prepare for emergencies.

People looking for a booster shot of a Covid-19 vaccine probably don’t need to fret about what brand it is: Many combinations of shots are likely to provide strong protection, according to a large new study.

In a comparison of seven different vaccine brands, British researchers found that most of them prompted a strong immune response, with the mRNA shots from Moderna and Pfizer-BioNTech eliciting the largest responses. The study was published on Thursday in The Lancet.

It’s too soon for researchers to say much about how well different vaccine boosters will work against the new Omicron variant, which has mutations that may allow it to evade some of the antibodies produced by existing Covid-19 vaccines.

All of the study’s 2,878 volunteers initially received two shots of either the AstraZeneca or Pfizer vaccines.

The researchers then tested seven different vaccines as boosters: along with AstraZeneca and Pfizer, they tried three brands that have been authorized in various countries: Johnson & Johnson, Moderna and Novavax. They also tried two shots that have not been authorized anywhere: an mRNA vaccine from CureVac, and a vaccine from Valneva made from inactivated coronaviruses.

After four weeks, the researchers collected blood samples from the volunteers and measured their antibody levels.

Most of the boosters used in the study raised antibodies to a level that would be the equivalent of at least 90 percent protection against infection. And the mRNA vaccines by Pfizer and Moderna produced much higher antibody levels than the other vaccines did.

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WASHINGTON — The Centers for Disease Control and Prevention plans to toughen coronavirus testing and screening of international fliers to the United States by requiring them to provide a negative result from a test taken within 24 hours of departure, a spokesman for the agency said Tuesday night.

The move to strengthen the testing regime comes amid growing concern about the Omicron variant, a highly mutated form of the virus that was first documented by researchers in South Africa and detected in more than a dozen countries around the world.

“C.D.C. is working to modify the current global testing order for travel as we learn more about the Omicron variant; a revised order would shorten the timeline for required testing for all international air travelers to one day before departure to the United States,” said the spokesman, Jason McDonald.

“This strengthens already robust protocols in place for international travel, including requirements for foreign travelers to be fully vaccinated,” he added.

Earlier Tuesday, a senior administration official, who spoke on the condition of anonymity because no final decisions had been made, said health officials were examining whether current testing — which allows fully vaccinated people to take a P.C.R. test up to three days before departing on a flight to the United States — was stringent enough. It was unclear whether the new 24-hour rule would require a rapid P.C.R. or antigen test.

President Biden has said he will announce on Thursday his plans for enhancing the fight against the pandemic. It is not clear whether he will announce tougher testing requirements for international travelers, which was first reported by The Washington Post, during Thursday’s speech. Mr. McDonald offered no timeline for the C.D.C.’s action.

In the meantime, Mr. McDonald said, the C.D.C. continues to recommend that all travelers get a coronavirus viral test three to five days after arrival in the United States. Unvaccinated travelers should self-isolate and quarantine for seven days after arrival, even if they test negative.

Natalie Quillian, the deputy coordinator for the Covid response at the White House, said in an interview on Monday that the Omicron variant had prompted the administration to re-evaluate its efforts to ensure that people entering the country were not carrying the virus.

“We’re constantly looking at what can we do to make that travel system even stronger,” she said. “For example, right now we’re assessing all of our tests to make sure they’re effective in picking up this variant.”

She added: “If we found that a test was not effective at picking up the variant, we would remove that from the list that is accepted to enter the country.”

A new 24-hour testing rule could make visiting the United States difficult, some travelers said. Paula Tolton, 23, a student in Taipei, Taiwan, said the plan made her worry about missing the flight she planned to take next month to visit her family in Jacksonville, Fla.

Even the current 72-hour rule is nerve-racking, she said. “I’ve had that stress before when a P.C.R. test didn’t come back when I was supposed to fly here in April,” she said. “I was freaking out.” Finding a clinic that could meet the 24-hour mark would make traveling back home much harder, she added.

A federal advisory committee on Tuesday voted to recommend that the government for the first time authorize the use of an antiviral pill to combat the worst effects of Covid-19.

The advisory committee, in a surprisingly narrow 13-to-10 vote, endorsed the pill from Merck, while public health officials worldwide raced to buttress their defenses against the newly emerging Omicron variant of the coronavirus.

The Merck treatment, known as molnupiravir, has been shown to modestly reduce the risk of hospitalization and death from Covid. The pill could be authorized for use in the United States within days and available to patients within weeks.

In the coming weeks, the F.D.A. may also authorize a similar pill from Pfizer that appears to be significantly more effective than Merck’s. Together, the arrival of the two easy-to-use treatments could provide a cushion against a resurgent virus.

The F.D.A. advisory panel, a group of experts on antimicrobial drugs, recommended that Merck’s treatment be authorized for people with Covid who are at high risk of becoming severely ill. That would most likely cover tens of millions of Americans who are older or have medical conditions such as obesity, diabetes or heart disease.

But the committee’s close vote reflected doubts about the pill’s effectiveness and concerns that it could cause reproductive harm.

“The efficacy of this product is not overwhelmingly good,” said Dr. David Hardy, an infectious disease physician in Los Angeles. Still, he voted to recommend the drug, saying “there is a need for something like this.”

Other members of the committee who voted against authorization said more research was needed about the drug’s safety. “The risk of widespread effects on potential birth defects, especially delayed effects on the male, has not been adequately studied,” said Dr. Sankar Swaminathan, an infectious disease specialist at the University of Utah.

The pills, which doctors will prescribe and will be dispensed at pharmacies, are much more convenient and are expected to reach many more people than the monoclonal antibody treatments that have typically been used to aid high-risk Covid patients. The antibody treatments are expensive and typically given intravenously at hospitals or clinics.

The Biden administration has been hoping that the emergence of the antiviral pills from Merck and Pfizer will help end the most acute phase of the pandemic. The U.S. government has spent billions of dollars to secure millions of treatment courses of the new pills.

Merck’s clinical trials primarily enrolled people who were infected with the Delta, Mu and Gamma variants of the coronavirus. Scientists have yet to run experiments to see how well the pills block Omicron viruses from replicating. But there are reasons to think they would remain effective even if the variant can sometimes evade vaccines, as well as monoclonal antibodies.

A federal judge issued a preliminary injunction on Tuesday to halt the start of President Biden’s national vaccine mandate for health care workers, which had been set to begin next week.

The injunction, written by Judge Terry A. Doughty, effectively expanded a separate order issued on Monday by a federal court in Missouri. The earlier one had applied only to 10 states that joined in a lawsuit against the president’s decision to require all health workers in hospitals and nursing homes to receive at least their first shot by Dec. 6 and to be fully vaccinated by Jan. 4.

“There is no question that mandating a vaccine to 10.3 million health care workers is something that should be done by Congress, not a government agency,” Judge Doughty of U.S. District Court for the Western District of Louisiana wrote. He added: “It is not clear that even an act of Congress mandating a vaccine would be constitutional.”

The judge, who was nominated to the court by former President Trump, also wrote that the plaintiffs have an “interest in protecting its citizens from being required to submit to vaccinations” and to prevent the loss of jobs and tax revenue that may result from the mandate.

Several cities and states had already imposed their own vaccine mandates for health care workers, in an effort to contain outbreaks that were often passed from communities into medical settings like nursing homes. The momentum for vaccine mandates gained steam during the summer as the Delta variant swept through nursing homes, causing spikes in staff and resident infections, as well as overwhelming hospitals in many states with another Covid surge.

Some of the larger hospital chains and several big nursing home operators also began requiring staff vaccinations, before the president began calling for nationwide compliance. Vaccinations among health care employees have increased since the summer, although cases among residents and staff remain in the thousands reported each week. Nationwide, the immunization rates among nursing home staffs is more than 74 percent, although much lower rates still exist in some regions.

In leading a 14-state lawsuit against the mandate, Attorney General Jeff Landry of Louisiana said the federal mandate would blow holes in state budgets and exacerbate shortages in healthcare facilities.

The Biden administration tied compliance with the vaccine mandate to federal funding, requiring immunizations of millions of workers at hospitals, nursing homes or other health facilities that heavily rely on the Medicare or Medicaid programs. But many health care providers — especially nursing home and rural hospital operators — complained that staff members who were hesitant to be immunized would leave, aggravating employee shortages that plagued the industry long before the pandemic.

Those complaints helped swell opposition in many states, like Texas and Florida, that have been vehemently against dictates on vaccines, mask-wearing and other federal policies at the heart of public health advice during the pandemic.

More than a dozen states and some employers joined forces to fight a broader mandate that would require private employers of 100 or more workers to impose company-wide immunization. An appeals court has temporarily blocked that mandate as well, as the challengers to the policy pursue their arguments that the Occupational Safety and Health Administration overstepped its authority.

In response to recent court decisions, the Centers for Medicare & Medicaid said in a statement, “While we cannot comment on the litigation, CMS has remained committed to protecting the health and safety of beneficiaries and health care workers. The vaccine requirement for health care workers addresses the risk of unvaccinated health care staff to patient safety and provides stability and uniformity across the nation’s health care system.”

The injunction issued on Tuesday is a first step in the lawsuits against the vaccine mandate. The cases still have to be argued before a judge, and any lower-court ruling will likely be appealed.

Los Angeles Lakers star forward LeBron James has entered the N.B.A.’s coronavirus health and safety protocols, the team announced Tuesday.

James missed the team’s game Tuesday night against the Sacramento Kings. It is unclear when he will be able to play again, and it is also unclear whether James has tested positive for the coronavirus or has come into close contact with someone who tested positive.

Lakers Coach Frank Vogel told reporters before the game that he found out Tuesday morning that James had entered the health and safety protocols. Vogel said the team arranged for James to get “back to L.A. safely.”

Players who test positive for the coronavirus must miss at least 10 days before they can return. All players who enter the protocol must return two negative tests on separate days before being allowed to play again.

James, 36, said before the season that he had been vaccinated against Covid-19.

Typically, players who are vaccinated face less stringent requirements than unvaccinated players. After Thanksgiving, though, the league implemented enhanced testing requirements even for vaccinated players, according to documents sent by league officials to each of the 30 teams. They did so with the expectation that the holiday would increase players’ potential exposure to the virus.

“We just want the best for him right now,” Vogel said. “That’s where our thoughts are. We have a next man up mind-set. It’s an 82-game season. You got to deal with guys being in and out of the lineup. We’ve been without him some already this season.”

This will be the 12th game James has missed this season. He missed 10 because of ankle and abdominal injuries. He also missed the Lakers’ game against the Knicks earlier this month after being suspended for the first time in his 19-season career.

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Top federal health officials said on Tuesday that they were expanding a surveillance program at some of the largest U.S. airports as part of a sprawling effort to identify and contain what could be the first cases of the Omicron coronavirus variant in the United States.

Dr. Rochelle P. Walensky, the Centers for Disease Control and Prevention director, said at a White House news conference on the pandemic that the agency was “actively looking” for the variant but had not found a case so far among the many positive virus samples sequenced around the nation each week. Cases of the Delta variant, which drove a devastating summer surge, still make up 99.9 percent of those samples.

Four international airports — in New York, Atlanta, Newark and San Francisco — would enhance screening in a search for possible Omicron cases. “This program allows for increased Covid testing for specific international arrivals, increasing our capacity to identify those with Covid-19 on arrival to the United States,” she said.

When asked whether President Biden planned to consider tightening recently relaxed restrictions on travel between the United States and Canada given the Omicron variant, the White House press secretary, Jen Psaki, told reporters separately on Tuesday that all decisions would be based on the recommendations of the president’s medical advisers. She said those advisers had not recommended new restrictions.

The White House last week announced a ban on travel from eight countries in southern Africa, a move questioned by some global health experts who said it amounted to a kind of punishment of South Africa for its transparency.

The new variant, which carries a startlingly high number of mutations, has caused fears among scientists and health officials across the globe about a more transmissible virus less susceptible to vaccines. Dr. Anthony S. Fauci, the government’s top infectious disease expert, reiterated at the news conference with Dr. Walensky that it was still too early to truly understand how dangerous the variant might be. Mr. Biden on Monday said the variant was “a cause for concern, not a cause for panic.”

It would likely be weeks before scientists studying the new virus were able to determine more about its properties, Dr. Fauci cautioned again on Tuesday. “We are hoping, and I think with good reason to feel good, that there will be some degree of protection,” from available vaccines, Dr. Fauci added. Asked about reports that the variant was causing only mild illness in younger people, he warned: “Be careful about bread crumbs. They may not tell you what kind of loaf of bread you have.”

Dr. Walensky said that the C.D.C. was examining ways to make international travel safer, possibly by testing for the virus closer to a traveler’s flight and “additional post-arrival testing and quarantine.” She said the C.D.C. was working with airlines to collect information on passengers that can be used for contact tracing if a case of Omicron is discovered.

Dr. Walensky also described an ongoing domestic effort to identify initial cases of the variant, saying that the C.D.C. was holding regular calls with local health officials, public health organizations and state laboratories, which help to sequence samples.

The United States had already made substantial progress this year in scaling up the number of virus samples examined for possible worrisome variants, she said, sequencing roughly 80,000 samples each week and one in seven positive P.C.R. test samples, a volume that suggests it might not be long before scientists find the virus.

Dr. Fauci and Dr. Walensky continued to urge people to get their boosters, which they said would provide more protection in the face of the new variant. Jeffrey D. Zients, the White House’s Covid-19 response coordinator, said that over 100 million fully vaccinated American adults were eligible for the doses but had not yet received one.

A surge of protective antibodies after a booster shot would likely still be formidable against Omicron, helping to prevent severe illness, Dr. Fauci said, even though the vaccine was developed to fight off the original form of the coronavirus.

Mr. Zients said that the federal government was already thinking about what a vaccination campaign with a newly formulated shot might look like, as pharmaceutical companies studied the possibility. “This includes conversations about the most appropriate regulatory pathway for review and authorizations,” he said.

A preliminary review by federal regulators determined that virus tests used in the U.S. would be able to detect the variant, Mr. Zients added.

Dr. Janet Woodcock, the acting Food and Drug Administration commissioner, said in a statement Tuesday that the agency was monitoring the new variant, citing guidance regulators released earlier this year about how it would evaluate new variant-specific vaccines on an expedited timeline.

Two top federal vaccine regulators who recently departed the agency argued in a Washington Post opinion column this week that younger, healthier people with a less urgent need for a booster dose might be better off waiting for a retooled vaccine that more precisely targets a worrisome new variant.

Maggie Astor contributed reporting.

The head of the United Nations-backed global vaccine distribution program said on Tuesday that it had shipped more than 11 million doses in the last 24 hours, its busiest day of deliveries ever.

The leader, Seth Berkley, said the program had been asking donors and vaccine manufacturers for months “to give us better quality donations” and more information on when doses would arrive. That message, he wrote on Twitter, “is just starting to be heard.”

Covax, a multibillion-dollar alliance between international health bodies and nonprofits, was supposed to ensure that poor countries got access to coronavirus vaccines and that rich countries did not hoard them. It has struggled in that mission and had to scale back its goals.

The new Omicron variant has prompted rich, highly vaccinated countries like the United States, Britain and Japan to expand their booster programs, while poorer, less vaccinated countries like South Africa are still trying to get first doses to residents.

Only about 5 percent of people living in low-income countries have received even one vaccine dose, according to the Kaiser Family Foundation, a health organization that is tracking coronavirus infections and vaccine distribution.

“Getting doses to countries is the easy part,” Mr. Berkley wrote on Twitter. Making the doses readily available “is harder & requires active collaboration” among manufacturers, shipping companies and officials in those countries.

Two people who tested positive for the coronavirus in the Netherlands more than a week ago were infected with the Omicron variant, Dutch health officials reported on Tuesday.

The timing is significant because it suggests that the variant was already present in the country for at least a week before the arrival of two flights from South Africa on Friday, and before the World Health Organization labeled Omicron a “variant of concern,” the step that prompted countries around the world to ban flights from southern Africa, where researchers first identified the variant.

“We have found the Omicron coronavirus variant in two test samples that were taken on Nov. 19 and Nov. 23,” the Dutch health ministry said in a statement on Tuesday. “It is not yet clear whether these people had also visited southern Africa.”

The two samples were taken by municipal health services at public testing sites, and health authorities have started contact tracing in those areas, Dutch health officials said.

Although little is known yet about how transmissible Omicron is, or whether it can evade existing vaccines, its detection in Botswana and South Africa has created the most uncertain moment of the pandemic since the highly contagious Delta variant emerged in the spring.

The announcement from the Netherlands also highlighted that scientists still cannot say with certainty where or when the variant originated. So far, the first known sample of the Omicron variant was collected on Nov. 9 in South Africa, according to Gisaid, an international database for disease variants.

Officials across Europe fear that Omicron will add pressure on countries that are already in the grip of some of the worst coronavirus surges they have seen. Among them was France, which on Tuesday reported about 47,000 new cases over the past 24 hours and sharply rising hospitalizations — mostly thought to be driven by the Delta variant.

The European Center for Disease Prevention and Control said on Tuesday that so far, 44 cases of the new variant have been confirmed in 11 European countries. And in Britain, health officials announced at least 22 confirmed cases involving Omicron, including 13 in England and 9 in Scotland, bringing a new wave of tightened public health restrictions.

Andrea Ammon, the agency’s director, told an online news conference that all the confirmed cases in Europe have exhibited mild symptoms or none at all, and that authorities were analyzing six further “probable” cases. She said that health officials were conducting additional tests on people who have recovered from illness brought on by Omicron to help assess how the variant behaves in vaccinated people, and that more information was expected in a “couple of weeks.”

Countries across the European Union have scrambled to reinforce travel restrictions in the hope of curbing the spread of the heavily mutated variant, as the W.H.O. warned that the risk it posed was “very high.”

But the W.H.O.’s top official on Tuesday cautioned countries that their responses were “not evidence-based.”

“We still have more questions than answers about the effect of Omicron on transmission, severity of disease, and the effectiveness of tests, therapeutics and vaccines,” Dr. Tedros Adhanom Ghebreyesus said in Geneva.

The Dutch officials’ announcement about the two Omicron cases came after a chaotic series of closures in Amsterdam, which left some 600 passengers on two flights from South Africa stranded for a time on Friday. Some 61 passengers on those flights tested positive for the coronavirus, and at least 14 of them were found to be carrying the Omicron variant.

The Netherlands imposed tighter restrictions starting on Sunday in response to a Covid wave that began before Omicron was identified, ordering many businesses, including bars, restaurants and theaters, to close from 5 p.m. to 5 a.m. Dutch health officials reported more than 22,000 new coronavirus cases on Monday, one of the country’s highest daily totals since the pandemic began.

Reporting was contributed by Carl Zimmer, Adeel Hassan, Megan Specia and Aurelien Breeden.

Pfizer asked federal regulators on Tuesday to authorize a booster shot of its coronavirus vaccine for 16- and 17-year-olds, according to the company’s chief executive, Albert Bourla. The request is likely to mean that several million teenagers could quickly become eligible for an additional shot.

The Food and Drug Administration is expected to act promptly on Pfizer’s request, perhaps within a week, according to people familiar with the planning.

The agency authorized the vaccine, developed with BioNTech, on an emergency basis a year ago for everyone 16 and older, fully approving it for that age group in August. But at this point, those 18 or older are eligible for booster shots.

Recipients of Pfizer and Moderna vaccines are eligible for the additional injection six months after their second shot. Those inoculated with Johnson & Johnson’s vaccine, a one-shot regimen, are eligible for a booster two months after the shot.

According to data from the Centers for Disease Control and Prevention, 2.3 million 16- and 17-year-olds received their second dose of Pfizer’s vaccine at least six months ago. By the end of the year, the number is predicted to be about three million, or 36 percent of the population in that age group.

After months of angst and division among scientific advisers, the Biden administration’s booster rollout is now in full swing. All adults became eligible for booster shots on Nov. 19, although the C.D.C. only recommended them for people over 50 as well as those 18 and older living in long-term care facilities.

On Monday, the C.D.C. altered its guidance and urged all adults to get a booster shot either six months after their second dose of Moderna’s or Pfizer’s vaccine or two months after their shot of Johnson & Johnson. President Biden described boosters Monday as part of the administration’s strategy to combat the new variant, Omicron.

More than 40 million adults have gotten an extra shot since the F.D.A. first authorized the extra injections for select population groups in late September. At least another 90 million are eligible for boosters now, according to the C.D.C.’s data.

The South African drug maker Aspen Pharmacare said on Tuesday that it was finalizing the first agreement to control production of a Covid-19 vaccine in Africa.

The deal would allow Aspen to bottle and market the Johnson & Johnson vaccine across Africa under the brand name Aspenovax. Aspen would have the right to determine to whom the vaccine would be sold, in what quantities and at what price — but not to produce the actual contents of the vaccine. Instead, Johnson & Johnson would direct other facilities to make the ingredients to send to Aspen for the company to blend into vaccine doses.

Control over the intellectual property of Covid vaccines has become a point of increasing contention in the debate over how best to address the huge gap in vaccine access across African nations.

Aspen already bottles the Johnson & Johnson vaccine under a previous agreement, but as a contract manufacturer, it has had no say on where to ship the doses. Earlier this year, millions of doses bottled at Aspen’s plant in the city of Gqeberha were exported to other parts of the world at a time when many African countries had vaccinated fewer than 5 percent of their citizens. The arrangement generated harsh criticism after it was revealed by The New York Times, and the new agreement could avert a similar situation in the future.

Strive Masiyiwa, the African Union’s special envoy for Covid, said that getting to 70 percent vaccination coverage in Africa would require 900 million doses of vaccines.

The State Department’s coronavirus vaccine envoy is leaving her post after less than a year, at a time when the new Omicron variant is showing the peril of failing to protect large areas of the world from the virus.

The envoy, Gayle E. Smith, took a leave of absence from her job as chief executive for the ONE Campaign, an advocacy organization that seeks to eradicate poverty and preventable disease, to join the Biden administration in April. It was not clear on Tuesday why she was leaving the envoy post now, and she did not respond to a request for comment. People close to her said she stayed longer than the six months she had initially committed to the government position.

“She leaves behind a phenomenal set of accomplishments, a robust team and network who are prepared to carry our important work forward, and a comprehensive set of next steps to build on our progress,” Secretary of State Antony J. Blinken said in a statement on Tuesday announcing Ms. Smith’s departure.

He added that, “as Gayle has said many times, our work to defeat this pandemic and prevent future health threats is not over.”

The United States has donated more coronavirus vaccine doses to Covax, the global distribution program, than any other single country. During Ms. Smith’s tenure, the United States gave a total of 260 million vaccine doses to more than 110 countries and economic blocs, and Mr. Blinken said the United States was “well on our way” to the 1.2 billion doses that President Biden promised to deliver.

Still, only about 5 percent of people living in low-income countries have received even one vaccine dose, according to the Kaiser Family Foundation, a health organization that is tracking coronavirus infections and vaccine distribution.

Across Africa, about 8 percent of people have received one dose, leaving hundreds of millions of people vulnerable to new variants that might otherwise be repelled by vaccines. The emergence of the Omicron variant, which was first detected in southern Africa last week, is an example of what experts have warned for more than a year would happen if vaccines were not made readily available worldwide, rather than just in wealthy nations.

“We’re really far behind in achieving any level of equity” in vaccine distribution, said Jen Kates, the director of global health and H.I.V. policy at the Kaiser Family Foundation. “We’re far from it at this point.”

Ms. Kates said the Biden administration deserved some credit for trying to motivate other donors to deliver more doses to poorer countries, even as Mr. Biden made vaccinating Americans a top priority.

She noted that in many lower-income countries, vast hurdles remain in providing the cold storage and transportation necessary to “get shots in arms” after vaccine doses arrive.

“That’s going to take more financial resources and a more herculean effort than what has come so far,” Ms. Kates said.

Ms. Smith will be replaced for now by Mary Beth Goodman, a senior member of the State Department’s vaccine diplomacy office who has worked for years on global health, anti-corruption and economic programs during Democratic administrations. A permanent envoy will be named in coming weeks, officials said.

As Africa grapples with the new Omicron coronavirus variant, Xi Jinping, China’s leader, has pledged to deliver another billion doses of vaccines to countries on the world’s least vaccinated continent.

The announcement from Mr. Xi is part of China’s continuing effort to burnish its image as a responsible global power helping to fight the pandemic, and it comes at a crucial time for countries in Africa, especially the southern region, where Omicron was first documented. Scientists fear that Omicron could already be spreading rapidly there, but they cautioned that much about the variant remains unknown, including where it originated.

Health officials in South Africa said on Monday that Omicron appeared to be driving a new wave there. The daily average of new cases in the country has increased by more than 1,500 percent over the past two weeks, according to data from the Center for Systems Science and Engineering at Johns Hopkins University, although the case numbers remain far below the year’s earlier peaks.

Still, officials urged the public not to panic over the variant, and said it was still too soon to accurately assess whether it has a higher rate of transmission or causes more hospitalizations or severe illness.

With wealthy nations hoarding most of the global vaccine supply, Africa has the lowest vaccination levels of any continent, with just 10.3 percent of the population receiving at least one dose, compared with rates of at least 60 percent to over 80 percent in Europe, Asia, Latin America and the United States and Canada.

But in recent weeks, vaccine doses have started to flow into parts of Africa, and countries including South Africa — where nearly one-quarter of people are fully inoculated, one of the highest rates on the continent — are now dealing with the challenge of how to rapidly administer them. Shabir Mahdi, a virologist at the University of the Witwatersrand in Johannesburg, said that where doses are available, “countries are struggling to scale up.”

Mr. Xi’s announcement, made in a speech late Monday via video link at the opening of the Forum on China-Africa Cooperation, also appeared to be part of an effort to shift attention away from Beijing’s missteps in its early handling of the coronavirus crisis.

He said that 600 million of the one billion vaccine doses would be donated, and that the rest would be provided through other means, like joint production between Chinese companies and African countries. He also said that China would send 1,500 medics and public health experts to Africa.

China aims to help the African Union achieve its goal of vaccinating 60 percent of the continent’s population by 2022, Mr. Xi said.

Chinese officials had previously said that Beijing would make its vaccines affordable and give priority to Africa, where it has rapidly increased its investments in recent years. The new pledge of one billion doses comes after the more than 155 million shots that China had previously pledged to the continent. Of those, about 107 million have been delivered to 46 African countries so far, according to Bridge Beijing, a consultancy that tracks China’s impact on global health.

After Omicron emerged, The Global Times, a Chinese tabloid controlled by the Communist Party, boasted of China’s success in thwarting the transmission of the coronavirus, and said the West was paying the price for its selfish policies.

“Western countries control most of the resources needed to fight the Covid-19 pandemic,” the piece read. “But they have failed to curb the spread of the virus and have exposed more and more developing countries to the virus.”

Questions remain, however, about the efficacy of the Chinese-made vaccines. Several countries that had relied heavily on them to inoculate large parts of their populations were spooked by subsequent outbreaks this year.

Omicron adds to the uncertainty, as scientists around the world race to find out whether the current vaccines protect against it. Sinovac Biotech, one of China’s main vaccine producers, told The Global Times that it was also studying its vaccine’s effectiveness against Omicron.

GLOBAL ROUNDUP

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The prime minister of Greece announced on Tuesday that Covid shots would be obligatory for people ages 60 or older, and that those who failed to book a first shot by Jan. 16 would face monthly fines of 100 euros ($113).

About 500,000 people in Greece ages 60 or older have yet to be vaccinated, Prime Minister Kyriakos Mitsotakis told a cabinet meeting, adding that the revenue from the fines would go to state hospitals that have been stretched by the pandemic.

Describing the policy as “an act of justice for the vaccinated,” Mr. Mitsotakis said he had worried about penalizing people but hoped they would see the move as an act of “encouragement, not repression.”

“I felt a duty to stand by the most vulnerable, even if it might temporarily displease them,” he said.

Greece is averaging more than 6,400 new cases a day, among its highest numbers since the start of the pandemic, according to the Center for Systems Science and Engineering at Johns Hopkins University. With concerns that the winter holidays will lead to further spread, Mr. Mitsotakis said more free testing kits would be made available over the next two months.

More than 60 percent of Greece’s population is fully vaccinated. This month, Greece barred unvaccinated people from cinemas, theaters, museums and gymnasiums, joining a growing number of European nations imposing new restrictions on those who have not had Covid shots.

Here are other developments from around the world.

• Thirteen cases of the Omicron variant have been identified in England and nine in Scotland, the British and Scottish governments confirmed on Tuesday, bringing the total number of known cases in Britain to 22. Scotland’s first minister, Nicola Sturgeon, said the nine cases there were all linked to a single private event. Sajid Javid, Britain’s health secretary, said officials did not yet know whether all of the cases in England were linked to travel to southern Africa, raising concerns about potential community transmission.

• Covax, the global vaccine-sharing initiative, announced on Tuesday that it had allocated more than 4.7 million doses to North Korea, which is not believed to have administered any shots yet. The reclusive government has reported no coronavirus cases and has turned down several previous offers of doses, including from Covax, China and Russia. But in June, North Korea’s leader, Kim Jong-un, said that lapses in his country’s anti-pandemic campaign had caused a “great crisis,” according to state media.

• Cholendra Shumsher Rana, the chief justice of Nepal’s Supreme Court, tested positive for the coronavirus on Monday evening and has pneumonia, but is in stable condition, said Dr. Prabin Nepal, a spokesman for the Armed Police Force Hospital outside the capital, Kathmandu. Mr. Rana — a central figure in recent political turmoil in Nepal, a Himalayan country hit hard economically by the coronavirus — has been facing growing pressure to resign over corruption accusations, and court watchers in Nepal said there was a good chance he would not return to the bench.

• With the exception of passengers arriving from the United States, Canada is requiring all air travelers, vaccinated and unvaccinated, to be tested for Covid-19 at the airport and to isolate until results are received. The new measures are a response to the identification of the new Omicron variant of the coronavirus in Canada. The government also directed its vaccine advisory committee to review guidance on booster shots to address the new variant.

• Brazil detected its first confirmed case from the Omicron variant after a couple who recently traveled from South Africa to Brazil both tested positive, health officials said Tuesday afternoon. A second sample is being tested to confirm the presence of the variant, which would be the first confirmed in Latin America.

Reporting was contributed by Megan Specia, Mark Landler, Jin Yu Young, Bhadra Sharma, Vjosa Isai and Ernesto Londoño.

Regeneron said on Tuesday that its Covid-19 antibody treatment might be less effective against the Omicron variant of the coronavirus, an indication that the popular and widely beneficial monoclonal antibody drugs may need to be updated in case the new variant spreads aggressively.

The company said that previous laboratory analyses and computer modeling of certain mutations in the Omicron variant suggest that they may weaken the effect of the treatment. But studies using the variant’s full sequences have not been completed, it said.

The company said it had already been testing future antibody drug candidates, and that preliminary analyses indicated that some of those “may have the potential to retain activity against the Omicron variant.” More data is expected in the coming month, it said.

“What we have to admit is, in the course of the past six days, our urgency has increased,” Dr. George Yancopoulos, Regeneron’s president and chief scientific officer, told The Wall Street Journal in an interview. “What started out as a backup plan has now been made a lot more urgent.”

The Omicron variant has caused alarm among scientists because it contains mutations in the spike protein, the target of the government-supplied monoclonal antibody treatments made by Regeneron and Eli Lilly.

Scientists have also been scrambling to gather data on how effective the current vaccines will be against Omicron. Antiviral pills, including drugs from Merck and Pfizer that federal regulators are considering authorizing soon, are expected to hold up well against the variant because they target a different site of the virus from where Omicron’s mutations are clustered.

Monoclonal antibody treatments, given in a single infusion, use lab-made copies of the antibodies that people generate naturally when fighting an infection. They have been shown to significantly shorten patients’ symptoms. Regeneron’s cocktail reduces the risk of hospitalization by 70 percent.

The company said the treatment was effective against the Delta variant, which remains the dominant form of the virus in the United States.

Europe is once again at the epicenter of the pandemic. More cases are being reported each day than at any previous point in the pandemic. And governments have been forced to reimpose the types of strict restrictions that most Europeans thought were behind them.

The discovery of the Omicron variant has added urgency to European leaders’ efforts to curtail the surge. Cases of the new variant have so far been detected in travelers to more than 10 European countries, including Denmark, the Netherlands and Britain.

European leaders have tried to strike a balance between increasing caution and avoiding panic in responding to the new threat. But the winter surge has highlighted the disparities in vaccination rates across the continent. Although cases are rising in many countries, only those with the lowest vaccination rates are seeing deaths from Covid-19 reaching the levels that came after similar surges last winter.

Here is a closer look at where cases are rising in Europe:

Israel’s swift response to the discovery of the Omicron variant — including closing its borders to nonresident foreigners — was influenced in part by a government-wide “war game” held earlier in November, officials said.

In that exercise, senior officials simulated how they would respond to a fictional scenario that bore striking similarities to what is happening now.

In a daylong drill on Nov. 11, Israeli officials had to respond to a hypothetical new “Omega” virus strain that would be more resistant to vaccines and would spread to Israel from two foreign countries during the second half of November.

In the simulation, officials including Prime Minister Naftali Bennett decided to keep Israel’s borders open to tourists into December, only to find that by the later stages of the exercise, the country’s hospitals were overwhelmed with patients. The correct decision, the participants concluded afterward, would have been to close Israel’s borders to most foreigners immediately, according to Yaacov Ayish, a retired general who helped plan the drill.

“It was one of the lessons,” Mr. Ayish said. “Suddenly, all the government agencies and the military had to analyze it as an option.”

The outcome was one of the factors that influenced Israel’s real-life decision on Saturday night to bar all foreign visitors, a move it made before any other country, said Keren Hajioff, a spokeswoman for Mr. Bennett.

Throughout the drill, participants were shown fictional television news reports to help set the scene for the next stage of the simulation. The participants in the drill drew at least one more conclusion that is reflected in Israel’s response: Officials realized that the stock of P.C.R. tests on hand at that time might not be advanced enough to detect future variants of the virus.

That helped prompt the Israeli government to order millions of higher quality P.C.R. tests, which are now being used to screen for Omicron, Ms. Hajioff said.

Israel’s cabinet on Sunday also granted Shin Bet, the domestic intelligence agency, temporary permission to access the phone data of people with confirmed cases of Omicron in order to trace who those people met recently. The agency was given similar powers during earlier waves of the pandemic.

The Omicron variant gained world attention a week ago, when researchers in southern Africa detected a version of the coronavirus that carried 50 mutations.

Thirty of these mutations are on the spike protein — arguably the most important part of the virus — and of those, 26 were unique mutations we hadn’t seen before. By contrast, the Delta variant had 10 unique mutations and Beta had 6.

When scientists look at coronavirus mutations, they worry about three things: Is the new variant more contagious? Is it going to cause people to get sicker? And how will the vaccines work against it?

This episode of “The Daily” explores when there will be answers to these three questions, and looks at the discovery of the variant and the international response to it. Listen below:

Among the many unknowns surrounding the new coronavirus variant called Omicron, named after the 15th letter of the Greek alphabet, one has stood out to many English speakers: How is it pronounced?

There is no single, agreed-on English pronunciation, experts say.

One pronunciation, according to Merriam Webster, is “OH-muh-kraan,” with a stress on the first syllable.

A World Health Organization official, Dr. Maria Van Kerkhove, recently said it that way when announcing that the variant was of concern.

In the United States, it is often pronounced “AH-muh-kraan,” Merriam Webster says. Less common are “OH-mee-kraan,” as Prime Minister Boris Johnson of Britain pronounced it this week, or “OH-my-kraan.”

On the New York Times podcast “The Daily,” Apoorva Mandavilli, who reports on the coronavirus and its variants, said she was going with “AH-muh-kraan.”

“I don’t think it really matters that much, honestly,” she said.

The New Oxford English Dictionary gives a pronunciation that differs from those in Merriam-Webster, according to Dr. Andreas Willi, a comparative linguistics professor at Oxford University. “Namely rather like an English phrase ‘o-MIKE-Ron,’” he said.

The word is a compound from the Greek “o mikron,” meaning “small o.” In classical Greek, the word was pronounced with the second syllable sounding like an English “me,” Dr. Willi said.

Peter Sokolowski, editor at large at Merriam Webster, said that because the Greek word is transliterated for pronunciation into English, sounding much as the word “omnipotent” is different from its Latin “omni-potent” origin, then the “AH-muh-kraan” pronunciation “makes perfect sense.”

But, he added, “There isn’t a wrong answer.”

“The question of British versus American pronunciation of the first syllable isn’t really specific to this particular word,” Dr. Willi said. “Compare the British versus American pronunciation of ‘god.’”

The divergences are to do with the name having been adopted as a loanword and used by English speakers in different places at different times, Dr. Willi said.

“When we speak of ‘Paris’ in English, that is also very different from the ‘proper’ French way of pronouncing the same name,” he said. “But it is hardly wrong in a strict sense.”

The Omicron variant has stirred alarm in India, which was hit hard this year by a devastating Covid wave fueled in part by another variant.

The new variant has forced the Indian government to review its decision to resume scheduled international flights beginning on Dec. 15. The flights had been stopped when Prime Minister Narendra Modi ordered a nationwide lockdown in March 2020, though some resumed after it established air travel bubbles with several nations.

While experts say it will most likely be weeks before more is known about Omicron’s transmissibility and the severity of the illness it produces, countries have scrambled to introduce new travel restrictions to halt its spread.

On Monday, Mr. Modi held an emergency meeting to review India’s travel rules. The country had only recently resumed issuing tourist visas as it reported the lowest daily cases since the pandemic began. India has also restarted exports of vaccines manufactured domestically.

Hospitals in New Delhi, the capital, where the earlier wave driven in part by the Delta variant shook the health care system, have been asked to remain on high alert. New Delhi’s top elected official, Arvind Kejriwal, asked Mr. Modi’s government to halt all flights from countries where the new variant had been found.

Instead, the Indian authorities reissued guidelines on Monday for travelers arriving from countries where cases of the Omicron variant have been reported. Passengers arriving from Europe, South Africa and other affected countries now face mandatory testing on arrival. They must quarantine at home for seven days after testing negative, and take another test on the eighth day.

Starting on Wednesday, the authorities said, they will require passengers to produce their travel history over the previous 14 days, along with results of a negative P.C.R. test before boarding any plane flying to India. Government officials said they had designated a hospital to treat and isolate any individual who tests positive for Omicron.

Officials in Mumbai, India’s financial capital, said that over the past 15 days, at least 1,000 travelers have landed in the city from African countries where Omicron has been detected.