The White House is extending travel bans on Europe and Brazil, and adding South Africa, because of the virus.

President Biden will ban travel by noncitizens into the United States from South Africa because of concern about a coronavirus variant spreading in that country, and will extend similar bans imposed by his predecessor on travel from Brazil, Europe and the United Kingdom, his press secretary said Monday.

The move comes as officials in the new Biden administration are trying to get their hands around a fast-changing pandemic, with public health officials racing to vaccinate the public — and to expand the supply of vaccine — as more contagious variants of the coronavirus spread.

Dr. Anthony S. Fauci, the government’s leading infectious disease specialist, said at the White House last week that “we’re following very carefully” the variant of the virus in South Africa because it appears to be more highly contagious. On Monday, Moderna said its vaccine is effective against new variants of the coronavirus that have emerged in Britain and South Africa. But the immune response is slightly weaker against the variant discovered in South Africa, and so the company is developing a new form of the vaccine that could be used as a booster shot against that virus.

And Dr. Rochelle Walensky, the new director of the Centers for Disease Control and Prevention, offered a blunt assessment of the vaccination campaign on Sunday, predicting that supply would not increase until late March. Federal health officials and corporate executives agree that it will be impossible to increase the immediate supply of vaccines before April because of lack of manufacturing capacity. A third vaccine maker, Johnson & Johnson, is expected to report the results of its clinical trial soon.; if approved, that vaccine would also help shore up production.

“I can’t tell you how much vaccine we have, and if I can’t tell it to you then I can’t tell it to the governors and I can’t tell it to the state health officials,” she told “Fox News Sunday.”

Mr. Biden’s travel ban is a presidential proclamation, not an executive order; typically, proclamations govern the acts of individuals, while executive orders are directives to federal agencies. It will go into effect Saturday and apply to non-U.S. citizens who have spent time in South Africa in the last 14 days. The new policy, which was earlier reported by Reuters, will not affect U.S. citizens or permanent residents, officials said.

On his last full day in office, Mr. Trump tried to eliminate the Covid-19-related ban on travel the United Kingdom, Ireland, 26 countries in Europe and Brazil, saying it was no longer necessary. Jen Psaki, now the White House press secretary, said at the time that ending the ban was the wrong thing to do; on Monday, she announced during her regular briefing that it would remain intact.

“With the pandemic worsening and more contagious variants spreading, this isn’t the time to be lifting restrictions on international travel,” she said.

Ms. Psaki also said the Biden administration intends to hold regular public health briefings three times a week, beginning this Wednesday. She said Mr. Biden would be “briefed regularly” on the pandemic, adding, “I suspect far more regularly than the past president.”

The variants of the coronavirus that are now spreading in South Africa and Brazil have not yet reached the United States. But on Monday health officials announced a case of the variant found in South Africa had been recorded in New Zealand in a returned traveler who had been released from hotel quarantine after twice testing negative. Over two dozen countries have now reported cases of the variant.

In addition to the travel bans, Mr. Biden issued an executive order last week requiring that all international travelers present negative coronavirus tests before leaving for the United States. The move extended a C.D.C. requirement for the tests that was issued by the Trump administration but set to expire on Tuesday.

A White House official said Sunday that the C.D.C. will not issue waivers from that policy as some airlines had requested.