Biden’s New Virus Plan Aims to Keep Economy Open as Omicron Spreads

President Biden, confronting a worrisome new coronavirus variant and the potential of a winter surge, laid out a new pandemic strategy on Thursday that includes hundreds of vaccination sites aimed at families, booster shots for all adults, new testing requirements for international travelers and free at-home tests that will be covered by private insurers or available at community health centers.

The push to expand access to at-home testing is a tacit acknowledgment by the White House that vaccination, which the president has touted as the path out of the pandemic, is not enough on its own. Experts have argued for months that mask-wearing and testing are also essential, and the need for testing will become even more urgent if the new Omicron variant is found to evade protection from vaccines, which has not yet been established.

“We’re going to fight this variant with science and speed, not chaos and confusion,” the president said at the National Institutes of Health.

Mr. Biden’s announcement comes a day after Omicron was detected in the United States for the first time, in California. On Thursday, a second case was detected, in a Minnesota resident who had recently traveled to New York City.

Conquering the pandemic — or at least bringing it under control — is by far Mr. Biden’s most daunting task as president, and it is especially complicated because it has become so divisive. The president struck a theme of unity in his remarks, urging Americans to come together around his plan. White House officials, and president himself, have said the plan is aimed at keeping the economy and schools open.

“We moved forward in the face of Covid-19 Delta variant, and we’ll move forward in the face of Omicron variant as well,” he said. “And we’ll do it by keeping the faith and doing it together as the United” — and here, Mr. Biden paused for emphasis — “States of America.”

Yet on Capitol Hill, Republicans threatened to shut down the government over Mr. Biden’s vaccine rules for large employers, even as the president spoke. Vaccine mandates have been held up in court, and Mr. Biden took pains to note that his current plan did not rely on lockdowns or vaccination requirements.

The president is also imposing new testing rules on international travelers to the United States, requiring them to present evidence of a negative test within a day before departure. He said the Centers for Disease Control and Prevention is studying alternatives to quarantining in schools, including “test to stay” policies where exposed students remain in school, wear masks and test again to avoid infection. And he promised the free at-home tests would be available next month.

“The bottom line is this winter, you’ll be able to test for free in the comfort of your home and have some peace of mind,” Mr. Biden said.

Experts said the announcement on testing was particularly important.

A big part of the plan is a renewed push to get people vaccinated, including the latest group to become eligible, children ages 5 to 11. The administration plans to launch “hundreds of family vaccination clinics” that will offer vaccinations and boosters for people of all eligible ages, according to a fact sheet provided by the White House.

The plan also includes a national campaign to reach the 100 million Americans who are eligible for boosters and have not had them. The campaign will include paid advertising and free rides to vaccination sites coordinated by AARP, the advocacy group for older Americans. And the Federal Emergency Management Agency will launch what the administration is calling Family Mobile Vaccination Clinics, beginning with deployments to Washington and New Mexico. The goal is for states and localities to replicate the model “with full federal funding and support,” officials said.

Mr. Biden called on employers to provide paid time off for employees to get boosters.

The new strategy will extend the current mask mandate for people on airplanes, trains and buses, and in terminals and transit hubs, through mid-March.

Mr. Biden ran for office on a promise to get the pandemic under control. Since then, though, the virus “has thrown us a number of curveballs, and unfortunately they were thrown at 200 miles an hour,” said Michael T. Osterholm, who directs the Center for Infectious Disease Research and Policy at the University of Minnesota.

Many experts predict a U.S. winter surge, regardless of whether Omicron spreads widely in the country. The nation has been reporting an average of more than 80,000 new cases a day over the last few weeks, according to a New York Times database; six months ago, the average was roughly 12,000 new cases a day.

Much remains unknown about Omicron, which was first spotted by scientists in southern Africa and now known to be present in more than 30 countries. It has mutations that scientists say may allow it to spread more quickly and cause more breakthrough infections in vaccinated or previously infected people, though neither characteristic has yet been confirmed.

Under the president’s plan, at-home tests would be reimbursed for the 150 million Americans with private insurance. To ensure access for those who lack insurance, or who are covered by Medicaid, the administration intends to distribute an additional 15 million tests to community health centers and rural clinics.

Looking forward, experts envision a world where people will test themselves as soon as they exhibit symptoms — and then, if they are positive, would go into quarantine and seek treatment with new antiviral medicines that are in development. Early testing is important because the antivirals work best just after the onset of symptoms. The White House says it is taking steps to secure 13 million courses of antiviral treatments.

In the United States, home tests have been relatively hard to come by because of supply shortages, and they are expensive — as much as $25 apiece. Dr. Carlos del Rio, an infectious disease specialist at Emory University, said that rather than have people go through the cumbersome process of seeking insurance reimbursement for tests, “we should just subsidize them and make it incredibly cheap.”

In Britain, he noted, rapid tests are free, and in Germany they cost consumers about $1 apiece.

Reimbursement for at-home tests in the United States will not happen immediately, and will not be retroactive, the senior administration officials said, adding that federal agencies would issue guidance by Jan. 15 to clarify that insurers would have to reimburse people for at-home tests during the Covid-19 public health emergency.

It was unclear how many tests a person could be reimbursed for buying, or how the reimbursement would work. Dr. Shah, of the state health officials association, said his group would prefer that people get reimbursed when they buy the tests in pharmacies, as opposed to having to file later for reimbursement.

Private insurers already cover the cost of tests administered in doctor’s offices and other medical facilities. At least eight at-home tests are on the U.S. market.

BERLIN — Germany announced tough new restrictions on Thursday to exclude unvaccinated people from much of public life, seeking to break a soaring fourth wave of the coronavirus pandemic and blunt the worrisome new Omicron variant.

The new rules, which stopped short of enforcing a complete lockdown on the unvaccinated, followed an agreement hammered out between Chancellor Angela Merkel, her successor, Olaf Scholz, and state governors.

Under the new rules, those wishing to go to bars and restaurants, or shop anywhere but in stores carrying basic necessities — like pharmacies or grocery stores — have to present proof of vaccination or documentation of recovery from a recent coronavirus infection. Some of those restrictions have been in effect already in some states; the agreement sets a uniform nationwide standard.

With the new rules, and a promise by Mr. Scholz this week that he would push a law making vaccinations mandatory, Germany is following the path of Austria, which recently mandated that all adults be inoculated by February. It comes as both countries contend with strident anti-vaccination sentiment in their populations that have kept vaccination rates low compared with other western European countries.

“You can see from the decisions that we have understood that the situation is very serious,” Ms. Merkel said at a news conference after a teleconference with state governors.

The announcement, which followed two meetings between federal and state governments in three days, came after the incoming government was criticized as not taking the Covid-19 crisis seriously enough. Though Ms. Merkel is still chancellor, Mr. Scholz was active at the meetings and vocal at the news conference afterward, underlining that the new rules and the establishment of a vaccine task force were supported by both the incoming and outgoing administrations.

“I am glad that in this difficult situation we are working shoulder to shoulder, that party politics is taking a back seat and the health of the citizens is the focus of the common endeavor,” said Mr. Scholz, who is expected to be sworn in next week.

Minnesota health officials said on Thursday that a man who lives in the state was infected with the Omicron variant of the coronavirus. The man, who officials said had recently traveled to New York City, represents the second known case of the variant in the United States.

Leaders in Minnesota said the discovery was unsurprising, and credited robust disease surveillance systems for finding it.

“This news is concerning, but it is not a surprise,” Gov. Tim Walz said in a statement. “We know that this virus is highly infectious and moves quickly throughout the world. Minnesotans know what to do to keep each other safe now — get the vaccine, get tested, wear a mask indoors, and get a booster.”

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.

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.

He was tested for the virus on Nov. 24. The state’s Public Health Laboratory determined late Wednesday night that he had the Omicron variant, officials said during a call on Thursday.

“Suffice to say, everybody around the country is looking for information and on alert” for the Omicron variant, said Jan Malcolm, the state’s health commissioner. “It’s very, very early hours.”

Minnesota officials said they were working with New York City and federal health officials to look into the case, and that the man was cooperating with case investigators, and following public health officials’ instructions to isolate himself.

One of his close contacts has since tested positive for Covid-19, 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.

On Wednesday, California health officials announced that a San Francisco resident had been infected with the Omicron variant — a finding they emphasized was inevitable, as they worked to contain alarm over the variant’s discovery in the United States.

Experts have said since the Omicron variant was first identified that it was only a matter of time before the variant made its way to the United States, and that once a case was detected, many more would probably soon be found.

“I’m personally surprised it took this long” for the first case to appear, Dr. Bob Wachter, a professor and chair of the department of medicine at the University of California, San Francisco, said on Wednesday. “Today it’s in California, tomorrow it’ll be several other states.”

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.

In Minnesota, Covid hospitalizations have been steadily climbing since the summer, straining health care systems and pushing health workers to the brink as intensive care units have filled with patients.

“The Minnesota surge at this point is Delta — we know that from our surveillance and sequencing work,” Ms. Malcolm said on Thursday. “We do hope this additional news underscores the urgency of paying attention and knowing Covid is very much still with us.”

Sixty-three percent of all Minnesota residents are fully vaccinated, according to C.D.C. data, and in Hennepin County, the figure is 67 percent.

Ms. Malcolm said that there were no statewide pandemic restrictions in effect for businesses in Minnesota, nor any statewide mandate that people wear masks in public indoor settings. She added that she did not believe that any “further regulatory approaches” would be necessary to manage the spread of the Omicron variant. But she urged Minnesotans to take precautions voluntarily: Mask up while indoors, and get vaccinated.

“We feel like we’ve got the tools to manage this and stay ahead of this,” she said.

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.

Ms. Hochul said the individual, a Minnesota resident who was vaccinated and experienced mild symptoms, 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.

In a statement, 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.”

The Omicron variant still has not been identified in New York, Ms. Hochul said, adding that New Yorkers should continue to wear masks and get tested, while unveiling a new public messaging campaign to encourage people to get booster shots.

Last week, Ms. Hochul declared a state of emergency to give the state more flexibility to respond to the rise in cases and staff shortages in health care facilities. Under an executive order that goes into effect on Friday, for example, hospitals that have less than 10 percent bed capacity will have to temporarily pause elective surgeries.

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.

Dr. Bassett steps into the role as parts of upstate New York grapple with a spike in coronavirus cases, while the state contends with the largely unknown Omicron variant during the cold winter months, when people are likelier to gather indoors and travel during the holidays.

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.”

Dr. Bassett, who ran New York City’s health department for four years until 2018, took the helm of an embattled Health Department that suffered from an exodus of health officials under former Gov. Andrew M. Cuomo, who repeatedly interfered in the department’s coronavirus response.

Dr. Bassett replaced Dr. Howard A. Zucker, who announced his resignation in September following political pressure over his role in potentially helping obscure the coronavirus death toll in nursing homes under Mr. Cuomo.

Appointed by Ms. Hochul, who replaced Mr. Cuomo after he resigned in August, Dr. Bassett faces the daunting task of restoring trust in the state’s public health apparatus among experts and New Yorkers alike.

Dr. Bassett won acclaim as the city’s health commissioner for her role in navigating the Ebola scare in 2014, as well as an outbreak of Legionnaire’s disease and the threat of Zika. She most recently served as director of Harvard’s FXB Center for Health and Human Rights.

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.

The French authorities confirmed the first cases of the Omicron version of the coronavirus in mainland France on Thursday, but their alarm remained focused on a surge of infections fueled by the Delta variant.

France reported nearly 50,000 new cases of the virus in 24 hours on Wednesday, the highest daily total since the spring. The number of reported cases per 100,000 people has soared from less than 100 to more than 300 over the past month.

“We need to anticipate — there are still a lot of uncertainties,” Jean-François Delfraissy, the head of the French government’s Covid-19 scientific advisory council, told BFMTV on Thursday, referring to Omicron. “But let’s not fight the wrong fight. The real fight, the real enemy, is the fifth wave with the Delta variant.”

The surge has alarmed French authorities, even though they have so far ruled out a return to lockdowns or business closures. Mr. Delfraissy said that cold had pushed people indoors and that social distancing was no longer being scrupulously followed. The average number of new hospital admissions, including in intensive care, has also increased by roughly 40 percent over the past weeks, according to official statistics.

But hospitalizations are still below the peaks seen in previous waves, thanks to a vaccination rate of 75 percent of the entire population, and Mr. Delfraissy said that if people exercised renewed vigilance — by avoiding gatherings, working from home when possible and wearing masks more often — France could be spared the worst outcomes.

“Christmas isn’t in danger if we are all careful,” he said.

The French government, which recently made all adults eligible for booster shots, has steered clear of mandating vaccines, arguing that coercion would be counterproductive. Olivier Véran, the health minister, told reporters this week that a more “powerful incentive” was the national health pass, which is required to access museums, restaurants, cinemas and other public venues.

The announcement of two Omicron cases on Thursday in mainland France has added to concerns, even though scientists are still trying to understand the threat posed by the variant.

The health authority for the Île-de-France region, which includes Paris, said that the Omicron variant had been found in a man in his 50s, who returned from Nigeria last week and tested positive for Covid-19 after disembarking from his flight, though he showed no symptoms. The man, who was not vaccinated, was isolating at home, officials said.

In eastern France, health authorities in the Grand-Est region said in a statement that Omicron had been detected in a woman in her 40s who returned from South Africa last week. The woman was vaccinated but experiencing symptoms, and has been isolating at home.

Previously, France had confirmed only one case of the Omicron variant, in the overseas department of Réunion, in the Indian Ocean.

Uncertainty over how dangerous Omicron really is had prompted the French authorities to halt flights with 10 countries in southern Africa, where the variant was first detected. Flights will be allowed to resume starting this weekend, but with some restrictions on travelers still in place. France will also require all visitors arriving from outside the European Union to provide a negative coronavirus test result, regardless of their vaccination status.

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.

Scientists in the United States and Britain believe they may have identified a key step in how the Covid vaccine developed by AstraZeneca and Oxford University may cause an extremely rare but serious blood-clotting disorder.

The AstraZeneca vaccine was expected to be the workhorse of the world’s vaccination efforts, thanks in part to pledges to manufacture it on a nonprofit basis during the pandemic and to make it available cheaply in poorer countries. The company and its partners have distributed more than two billion doses worldwide.

But reports of the rare side effect — an autoimmune response that has led to dangerous and sometimes fatal clotting in a small number of cases — were among a series of setbacks that tarnished the shot’s reputation and caused many European countries to limit its use. It has yet to receive authorization in the United States, although the company has said that it hopes to achieve approval soon.

“We hope our findings can be used to better understand the rare side effects of these new vaccines — and potentially to design new and improved vaccines to turn the tide on this global pandemic,” Prof. Alan Parker of Cardiff University’s School of Medicine, one of the researchers involved in the study, said in a news release.

The findings, which were published on Thursday, in the journal Science Advances. suggest that the problem is linked to the vaccine’s use of another, harmless virus — an adenovirus — to deliver a coronavirus gene into human cells, in order to train the immune system to recognize and battle the virus.

The shot is injected into muscle tissue, but the report suggests that if the adenovirus leaks into the bloodstream it can bind to a protein in the blood called platelet factor 4, or PF4, which is involved in the natural clotting process.That process, in turn, could lead to the release of antibodies against the protein, causing platelets to cluster and blood clots to form in very rare cases, the authors of the article said.

“With a better understanding of the mechanism by which PF4 and adenoviruses interact there is an opportunity to engineer the capsid, or outer shell of the vaccine, to prevent this interaction occurring,” Dr. Parker said.

The Johnson & Johnson vaccine also uses an adenovirus, and has also been linked to the rare blood clotting disorder. The Pfizer or Moderna shots are based on a different technology, one that does not involve a helper virus.

Concerns about the rare side effect first emerged in March, causing many European nations began to rethink the vaccine’s use in some age groups.

The reaction was first discovered by scientists in Germany and Norway, but how or why it took place had remained a mystery.

Public health experts have expressed concern that the rare vaccine-related reactions have fueled hesitancy, particularly in Europe, and continue to emphasize that the AstraZeneca vaccine’s benefits far outweigh the risks.

In Britain, where the vaccine first went into use in January, the National Health Service reports that “the risk of this extremely rare side effect is around 1 in every 100,000 first doses” whereas the benefit of one dose leads to an 80 percent reduction in deaths. As of Aug. 11, the side effect had been linked to 73 deaths in Britain, according to the country’s Medicines and Healthcare Products Regulatory Agency. Britain has given more than 25 million first doses of the vaccine.

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.

South Koreans who went overseas to travel or live abroad and were expecting to come back home without having to quarantine got an unpleasant surprise after the government announced exemptions would be halted for visitors from all countries, including fully vaccinated Korean nationals.

The new regulations threw a wrench in Christmas plans for many of these travelers, who will have to self-isolate over the holidays. Previously, such travelers who had been fully vaccinated for at least two weeks were exempted from quarantine.

South Korean officials announced the change in quarantine rules on Thursday, after the country’s first cases of the new Omicron variant were confirmed the night before. The 10-day quarantine requirement for all arrivals takes effect on Friday and will continue until at least Dec. 16.

The measures were aimed at forestalling community spread of Omicron, Jeong Eun-kyeong, the commissioner of the Korea Disease Control and Prevention Agency, told a news conference on Thursday. “We must strengthen measures for inbound travelers,” she said.

On Thursday the nation reported its record number of daily cases at 5,266. This is its worst wave of cases despite a vaccination rate of 79 percent, according to figures collated by the Our World in Data project at the University of Oxford. South Korea is also one of the nations that have barred arrivals from several African countries in response to South Africa’s initial detection of the Omicron variant.

Some South Koreans said they were confused and frustrated by the sudden change in regulations.

“This isn’t fair,” said Alisha Kim, a mother of two sons in college in the United States, who was about to return to Seoul. She said that she would not have made the trip to visit them had she known she would be required to quarantine when she returned.

“We all worked so hard to abide by the government regulations until now, and wore our masks and washed our hands, and now all of a sudden we have to quarantine,” she said.

South Korea introduced a “living with Covid” plan this month, easing some social distancing measures and limits on business operating hours. It will delay any further loosening of restrictions for at least four weeks.

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.

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.”

As recently as last week, many public health experts were fiercely opposed to the Biden administration’s campaign to offer booster shots of the coronavirus vaccines to all American adults.

But the Omicron variant is starting to change all that.

Scientists do not yet know with any certainty whether the virus is easier to spread or less vulnerable to the body’s immune response. But with dozens of new mutations, the variant seems likely to evade the protection from vaccines to some significant degree.

Many of the experts who were opposed to boosters now believe that the shots may offer the best defense against the new variant. The extra doses may slow the spread, at least, buying time for vaccine makers to develop an Omicron-specific formulation, if needed.

“Based on what we know about the potential for immune evasion, I would err on the side of giving the booster,” said Dr. Celine Gounder, an infectious disease specialist at Bellevue Hospital Center who had opposed the Biden administration’s boosters-for-all push.

The administration isn’t waiting for scientific consensus. Alarmed by the preliminary reports about Omicron, officials at the Centers for Disease Control and Prevention said on Monday that all American adults should receive booster shots.

But greater support for boosters among scientists may eventually complicate efforts to deliver limited supplies of the coronavirus vaccines to poor countries. The World Health Organization has said for months that the clamor for extra doses in rich countries was robbing poorer nations of the first doses they desperately need. The agency has not changed that position.

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.

WASHINGTON — Treasury Secretary Janet L. Yellen on Thursday said she believed it was time to retire the term “transitory” to characterize inflation as temporary and suggested that the Omicron variant of the coronavirus could prolong the problem of rising prices.

Speaking at an event sponsored by Reuters, Ms. Yellen said that over the summer it appeared that the pandemic was subsiding and that the economy would soon normalize. The spread of new variants, she said, has changed that calculus.

“Now the new variant, the Omicron variant, the pandemic could be with us for quite some time and hopefully not completely stifling economic activity but affecting our behavior in ways that contribute to inflation,” Ms. Yellen said.

Ms. Yellen’s remarks echoed those of Jerome H. Powell, the Fed chair, who said earlier this week that inflation was more than a short-term issue.

“I am ready to retire the word transitory,” Ms. Yellen said. “I can agree that that hasn’t been an apt description of what we are dealing with.”

The Treasury secretary said that it was too soon to say what impact Omicron would have on the economy, noting that it could snarl supply chains and fuel further inflation, but that if it dampened economic growth it could blunt price increases. She warned, however, that it could cause “significant problems.”

“We’re very uncertain at this point just how significant a threat it’s going to be and closely watching the scientific evidence on this that’s accumulating,” Ms. Yellen said. “Hopefully it’s not something that’s going to slow economic growth significantly.”

She added: “This could again exacerbate supply chain and inflation problems.”

Ms. Yellen, a former Fed chair, said that the central bank was committed to using its tools to contain inflation but said there was little it could do to ease clogged supply chains. She said that Mr. Powell’s suggestion this week that the Fed would consider speeding up its plan to withdraw financial support from the economy “makes sense.”

“What we don’t want to have develop is a wage-price spiral, in which inflation becomes its own self-reinforcing kind of phenomenon that would become chronic in the U.S. economy, something endemic,” Ms. Yellen said.