WASHINGTON — By the time President Biden was briefed on the emergence of a fast-moving new Covid variant on the morning after Thanksgiving, he had a choice to make — and little information to base it on.

In a secure conference call from a vacation compound overlooking Nantucket Harbor, the president listened as his health advisers told him that the highly mutated virus was far more concerning than other variants they had seen in recent months. It spread twice as fast as the dominant Delta variant and had the potential to evade treatments and vaccines.

Banning travel from southern Africa, where the variant was discovered last week, would not stop the coronavirus from finding its way to the United States, the officials told Mr. Biden, even though Britain and several other countries had announced similar restrictions. But the measures might slow the spread.

During the 30-minute briefing, Dr. Anthony S. Fauci, the president’s top medical adviser for the coronavirus, and other health officials acknowledged how little they knew about the threat, according to White House officials and others familiar with the discussion. But they concluded that even a potentially marginal benefit from a travel ban was worth the criticism that it was likely to generate from the affected countries, the officials said. Better to be criticized for something you do, rather than for something you don’t do.

A few hours later, as Mr. Biden ate lunch with his extended family at the Nantucket Tap Room, the White House issued a statement in his name announcing a ban on travel from eight countries in southern Africa, prompting outrage among leaders in that region — and from global health experts who questioned the benefits of the move, saying it was tantamount to punishing South Africa for being transparent about the virus.

“Here’s what it does: It gives us time. Gives us time to take more actions to move quicker,” Mr. Biden said at the White House on Monday morning as he called the new variant, named Omicron, “a cause for concern, not a cause for panic.”

The sudden arrival of Omicron represented a jarring, here-we-go-again moment for a weary and politically divided country after nearly two years of battling the pandemic. It also underscored the difficult position the president is in as he seeks to respond aggressively to yet another public health threat.

The scramble among White House and public health officials on Thursday night and Friday morning was a reminder that the United States remains vulnerable to a virus that is still spreading, unchecked through largely unvaccinated parts of the world — a problem that is well beyond the control of any global leader. And it once again highlighted the political dangers for Mr. Biden and his party if a new wave of infections derails the country’s economic recovery and return to some semblance of normalcy.

The president on Monday sought to reassure the public, ruling out a return to the kinds of nationwide “shutdowns and lockdowns” that ground economic and social life to a halt last year. Instead, he said, the administration would combat the new variant “with more widespread vaccinations, boosters, testing and more.”

Mr. Biden’s call for more vaccinations came as the Centers for Disease Control and Prevention on Monday altered its guidance and urged all adults to get a booster shot when they are eligible, six months after their initial Pfizer or Moderna doses or two months after their initial Johnson & Johnson vaccine. The agency had previously urged eligible people over 50 and those living in long-term care facilities to get a booster shot, but stopped short of saying that everyone should do so.

In addition, Pfizer and BioNTech will ask federal regulators this week to authorize their booster shot for 16- and 17-year-olds, according to people familiar with the companies’ plan.

Scientists were working to make sure current tests could accurately detect the new variant, officials said; the administration was working with manufacturers to modify their vaccines and booster shots, should that prove necessary, Mr. Biden said.

White House officials said that the president would outline a detailed strategy for fighting the coronavirus this winter when he visits the National Institutes of Health on Thursday.

But significant risks remain, including to the nation’s economy.

Jerome H. Powell, the Federal Reserve chair, plans to tell lawmakers on Tuesday that Omicron creates more economic uncertainty and the possibility of further inflation, according to a copy of his prepared remarks.

“Greater concerns about the virus could reduce people’s willingness to work in person, which would slow progress in the labor market and intensify supply-chain disruptions,” Mr. Powell plans to say.

In his remarks on Monday, Mr. Biden promised that he was “sparing no effort, removing all roadblocks to keep the American people safe.”

That pledge came as some Republicans seized on the existence of another variant to attack the president. The Republican National Committee issued a statement saying that “Biden failed to shut down the virus as he promised.” Representative Ronny Jackson of Texas, who served as President Donald J. Trump’s White House physician, suggested that Omicron was created by liberals eager to impose further Covid restrictions.

White House officials dismissed the political criticism. Natalie Quillian, the deputy Covid-19 response coordinator, said the potential dangers from the new variant were serious enough to prompt a flurry of meetings among officials from multiple agencies, calls with pharmaceutical companies and urgent messages to health officials in other countries.

“There was a sense of concern, a sense that this felt different from other variants,” Ms. Quillian said. “This had enough of the markers to differentiate itself in the level of concern we felt. We sort of kicked into action Thursday night and Friday.”

The new variant upended the Thanksgiving holiday for administration officials and top scientists, who had scattered across the country for celebrations.

The variant was identified by South African scientists on Thursday afternoon, as many U.S. officials were sitting down to dinner. Shortly before midnight, Dr. David A. Kessler, the chief science officer for the government’s coronavirus response, reached out to a South African partnership, which sent back a genomic sequencing report on the variant.

Dr. Fauci and Dr. Rochelle P. Walensky, the C.D.C. director, were in contact with their counterparts in South Africa late on Thanksgiving Day. Jeff Zients, the president’s Covid-19 response coordinator, and others spent most of the night making calls.

By Friday morning, it appeared that Mr. Zients was leaning toward travel restrictions, according to one person familiar with the deliberations. At 10:30, Mr. Zients, Dr. Fauci and other top scientists were briefed by the South Africans, including Tulio de Oliveira, a geneticist who helped identify the Omicron variant.

After Mr. Biden made the decision to impose the travel ban, State Department officials told diplomats in the affected countries, and administration officials began calling airlines to inform them of the change. From the beginning of the discussion late Thursday, it took about eight hours to issue the presidential directive.

“Even if we bought ourselves a little bit of time to understand this more, that was valuable,” Ms. Quillian said. “And this is an action that’s not permanent.”

For now, the travel restrictions are the president’s primary response.

Dr. Michael T. Osterholm, an infectious disease expert at the University of Minnesota who advised Mr. Biden during the presidential transition, said that while travel bans could help officials gain “situational awareness,” they offered only temporary benefits. He said he would not be surprised if the restrictions were soon lifted.

“It’s like in a crime scene,” Dr. Osterholm said. “When you go to a crime scene, what do the police do right away? They lock everything down so they can figure out what’s going on. But that doesn’t mean they are going to keep things locked down for the rest of the day or the rest of the week.”

But several public health experts expressed outrage at the bans, saying they punished South Africa for doing what the United States expected of other nations: tracking the coronavirus, identifying worrisome variants and making the information public.

“Travel restrictions are exactly the wrong incentive to give to countries when you want them to share data,” said Gregg Gonsalves, an activist and associate professor of epidemiology at Yale University. “You want them to be on the lookout for new variants, and you shut your borders?”

Mr. Oliveira warned on Twitter on Monday that because planes were no longer flying to South Africa, his lab might run out of some of the chemical components known as reagents that are needed to test for the variant.

“It will be ‘evil’ if we cannot answer the questions that the world needs about #Omicron due to the travel ban!” he wrote.

The new variant has again raised criticism that the Biden administration is not doing enough to vaccinate the rest of the world, though that effort is complicated by vaccine hesitancy in other nations.

South Africa has fully vaccinated only 24 percent of its population, according to the Our World in Data project at the University of Oxford. It has a better vaccination rate than most countries on the continent, but has asked vaccine makers to stop sending doses because of trouble getting shots into arms, in part because of distribution bottlenecks and hesitancy.

Elsewhere in Africa, the vaccination rate is much lower; in some countries, even health care workers have had trouble getting their shots. The W.H.O. reported last week that only 27 percent of health workers in Africa had been fully vaccinated.

The Biden administration has pledged to donate more than a billion doses to other nations; so far it has shipped 275 million doses to 110 countries.

“Now we need the rest of the world to step up as well,” the president said.

But activists and some global health experts said the administration needed to move faster, arguing that vaccine inequities were the reason for the emergence of the variant.

African officials on Monday criticized the global effort to provide vaccines to their countries, saying in a joint statement that their low vaccination rates were the result of a lack of consistent, reliable doses.

“The majority of the donations to date have been ad hoc, provided with little notice and short shelf lives,” they said in the statement. “This has made it extremely challenging for countries to plan vaccination campaigns and increase absorptive capacity.”

“This trend must change,” they added.