Biden is addressing the nation on Omicron.

President Biden addressing the nation on Monday about the worrisome new Omicron variant of the coronavirus, following a weekend of furious meetings between his top scientific advisers and their counterparts in South Africa, whose scientists detected the new variant.

Mr. Biden has already restricted travel from eight nations, including South Africa, from coming to the United States, a move that experts said would buy the United States time in determining how to respond. But it will likely be a week, possibly two weeks, before experts know more about the new variant, including whether it evades current vaccines.

Despite significant questions about the variant itself, countries around the world have rushed to defend against its spread, with a cascade of border closures and travel restrictions that recalled the earliest days of the pandemic. The variant has yet to be detected in the United States.

Mr. Biden was elected on a promise to bring the pandemic under control — a task that is proving easier said than done. Viruses are by nature wily creatures, dedicated to ensuring their own survival, and that is especially true of the virus that causes Covid-19. Just as Mr. Biden was about to declare “independence from the virus” on the July 4 holiday, the Delta variant swept across the United States, causing another wave of hospitalizations and deaths.

Now there is Omicron, discovered in southern Africa and designated by the World Health Organization on Friday as a “variant of concern.” There is still relatively little known about Omicron. It has mutations that scientists fear could make it more infectious and less susceptible to vaccines — though evidence to support those fears has yet to be established.

Mr. Biden is trying to project an image of calm, and to keep the country from panicking, while also ensuring that Americans remain vigilant by getting vaccinated and taking other steps to prevent the spread of the coronavirus, including masking and social distancing.

The emergence of the new variant is also stepping up pressure on Mr. Biden and his administration to do more to share vaccines with the rest of the world, though there are some complicating factors.

South Africa, whose scientists detected the variant, has fully vaccinated only 24 percent of its population, according to data from 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; it is having trouble getting shots into arms, in part because many people are hesitant to take it.

Some experts argue those vaccine inequities are the reason for the emergence of the variant.

“This is precisely what experts have been predicting was going to happen — that the extraordinary inequities and gaps between low income countries and high income countries creates this massive vulnerability and its going to continue to generate these dangerous variants,” said J. Stephen bbring Morrison, a global health expert at the Center for Strategic and International Studies in Washington. “That point is glaringly obvious and it’s painful.”

Mr. Biden’s top health advisers, including Dr. Anthony S. Fauci, spent much of the holiday weekend consulting with their South African counterparts. The White House said that Mr. Biden met on Sunday with members of his Covid response team, including Dr. Fauci, the nation’s top infectious disease expert.

With much still unknown about the Omicron variant, Dr. Fauci told the president that it would take approximately two more weeks to learn more about its transmissibility and severity, the White House said, but that “he continues to believe that existing vaccines are likely to provide a degree of protection against severe cases of Covid.”

Administration officials — including the president himself — are encouraging the public to maintain vigilance and safeguard public health through inoculations, masking indoors and distancing.

“We don’t know the level of severity of it; we don’t have enough information yet,” Dr. Fauci said in an interview over the weekend, adding, “This is all the more reason to get vaccinated.”

Appearing on morning talk shows on Sunday, Dr. Francis Collins, director of the National Institutes of Health, cautioned Americans that the emergence of Omicron and the uncertainty that surrounds it are reminders that the pandemic is far from over.

“I know, America, you’re really tired about hearing those things, but the virus is not tired of us,” he said. “And it’s shape-shifting itself.”

“Please, Americans, if you’re one of those folks who’s sort of waiting to see, this would be a great time to sign up, get your booster,” Dr. Collins said on Fox. “Or if you haven’t been vaccinated already, get started.”