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WASHINGTON — The Biden administration is looking at ways to toughen coronavirus testing and screening of people flying to the United States, including possibly requiring all fliers to provide a negative result from a test taken within 24 hours of departure, according to a senior administration official.

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 discovered by researchers in South Africa and has spread to more than a dozen countries around the world.

The 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 Covid test up to three days before departing on a flight to the United States — was stringent enough.

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.

Natalie Quillian, the deputy coordinator for the Covid response at the White House, said in an interview on Monday that the arrival of 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 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 plaintiffs, he added, also 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.

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.

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.

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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.

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.

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.

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.