The administration doesn’t yet believe new formulations of coronavirus vaccines will be necessary, but is already working with Pfizer Inc., Moderna Inc. and Johnson & Johnson on contingency plans, he said. He said he’ll issue a new strategy to combat a winter surge of coronavirus on Thursday, but that the U.S. will not have to undertake further lockdowns or shutdowns.

If new vaccines are needed, he said, “We will accelerate their development and deployment with every available tool.”

The new Covid-19 variant has now been detected in more than a dozen countries, though not yet identified in the U.S. On Friday, Biden imposed travel restrictions on eight nations in southern Africa, where omicron is believed to have emerged, but declined to say Sunday if he’d expand the limits to other places.

Doctors in South Africa have said symptoms of the new variant appear to be milder than the delta variant, which drove a surge in infections in the U.S. earlier this year. But they have cautioned that it’s too early to reach conclusions about whether omicron is more or less dangerous than predecessors.

Biden said it’ll be a few weeks until scientists know how effective vaccines are against omicron, but that his experts believe “the vaccines will continue to provide a degree of protection against severe disease.”

Read more: Doctor Who Saw Omicron Early Says Symptoms Milder Than Delta

Biden said it’s “almost inevitable” that the variant will turn up in the U.S. “at some point.”

He hailed South Africa for promptly reporting omicron to the rest of the world. “This kind of transparency is to be encouraged and applauded,” he said.

However, the U.S., U.K., European Union and other governments have rewarded that transparency by blocking most travel from South Africa. Some of Biden’s ex-advisers have criticized the bans as ineffective, but the president argued that it was due to the “significant number of cases” in the region and travel limits would buy time to vaccinate more Americans.

“It gives us time to take more actions, to move quicker, to make sure people understand you have to get your vaccine,” he said.

The president said he doesn’t believe his travel ban will discourage future reporting of variants.

Read more: Omicron Cases Seen Mild; Biden Cautions on Panic: Virus Update

Sam Scarpino, managing director of pathogen surveillance at the Rockefeller Foundation, said it’s not clear yet how much more transmissible or virulent the variant is.

“The landscape right now is more speculation than hard facts. One of the really important things to remember is this was picked up quite early,” he said.

Full border closures can delay a surge, but partial closures are ineffective, he said. Countries should focus more on other border measures, like mandatory testing at point of arrival.

“These kinds of porous border closures don’t work. What they do is they are punishing people for sharing data early,” Scarpino said. “What we really need to do is deliver resources, vaccines and other resources, to countries we know that are affected, like South Africa, and we need to invest heavily in ascertaining the spread.”

Read more: Stocks Rally as Wall Street’s Covid Fears Ease: Markets Wrap

Some other countries have moved even more aggressively than the U.S. to curtail travel, according to the Bloomberg Covid-19 Travel Tracker. Switzerland added a ban on travel from Australia, in addition to African countries. Singapore delayed the reopening of quarantine-free travel with three Mideast countries because they’re transfer hubs for travelers from Africa.

Japan will close its borders from Tuesday to new foreign arrivals and told its own citizens to isolate on arrival. Australia paused plans to ease its own border restrictions, while South Korea is reviewing its own Covid plans.

Still, omicron has already spread widely as scientists study it to determine whether experts’ worst fears — that it could skirt immunities provided by vaccines or recovery from the virus — will be realized.

Cases have been found in Canada, the U.K., Germany, the Netherlands, Belgium, Austria, Israel, Italy, Switzerland, Australia and elsewhere. It was also found in a Hong Kong quarantine hotel.

As for the U.S., “it’s almost assuredly here,” Ezekiel Emanuel, a Vice Provost for Global Initiatives at the University of Pennsylvania and former health adviser to the Obama administration and the Biden transition, told MSNBC on Monday.

Authorities and scientists don’t know how much more dangerous the variant is or how effective vaccines are against it, and cases are already poised to spike during the holidays, he said.

“We are definitely going to have a bump after Thanksgiving and another bump after Christmas, and if this omicron is floating around, we will have very substantial increases in the country,” he said.

U.S. health officials have called on Americans to get vaccinated or get a booster shot to shore up their protection. All American adults are now eligible for boosters six months after their second shot, or two months after receiving Johnson & Johnson’s one-dose vaccine.

–With assistance from Jenny Leonard and Jennifer Epstein.