WASHINGTON (AP) — President Joe Biden on Monday will formally reinstate COVID-19 travel restrictions on non-U.S. travelers from Brazil, Ireland, the United Kingdom and 26 other European countries that allow travel across open borders, according to two White House officials.

The officials, who spoke on the condition of anonymity to discuss the order, also confirmed Sunday that South Africa would be added to the restricted list because of concerns about a variant of the virus that has spread beyond that nation.

Top U.S. infectious disease expert Dr. Anthony Fauci called Biden's decision “prudent” Monday in a round of television interviews.

“We have concern about the mutation that’s in South Africa," Fauci told "CBS This Morning." "We’re looking at it very actively. It is clearly a different and more ominous than the one in the U.K., and I think it’s very prudent to restrict travel of noncitizens.”

Biden is reversing an order from President Donald Trump in his final days in office that called for the relaxation of the travel restrictions as of Tuesday. Trump's move was made in conjunction with a new requirement from the Centers for Disease Control and Prevention that all international travelers to the U.S. obtain a negative test for COVID-19 within three days of boarding their flight.

Biden's team had announced that he would reimpose the travel restrictions, but the addition of South Africa to the restricted travel list highlights the new administration’s concern about mutations in the virus.

The South Africa variant has not been discovered in the United States, but another variant — originating in the United Kingdom — has been detected in several states.

Fauci said there is “a very slight, modest diminution” of the effectiveness of COVID-19 vaccines against those variants but “there’s enough cushion with the vaccines that we have that we still consider them to be effective against both the UK strain and the South Africa strain.”

But he warned that more mutations are possible and said scientists are preparing to adapt the vaccines if necessary.

“We really need to make sure that we begin, and we already have, to prepare if it’s necessary to upgrade the vaccines,” Fauci said. “We’re already taking steps in that direction despite the fact that the vaccines we have now do work.” Fauci spoke on NBC’s “Today” show and “CBS This Morning.”

Reuters was first to report Biden’s decision to add South Africa to the list.

Last week, Biden moved to expand on the CDC requirement and direct that federal agencies require international travelers to quarantine upon arrival in the U.S. and obtain another negative test to slow the spread of the virus.