Biden is addressing the nation on Omicron.

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.”