U.S. President Joe Biden on Monday reinstated COVID-19 travel restrictions to Brazil, Ireland, the United Kingdom and 26 European countries. He also added South Africa to the list.

The travel ban also suspends entry to nearly all foreign nationals who have been in any of the countries on the restricted list at any point during the 14 days before their scheduled travel to the U.S.

Top U.S. infectious disease expert Dr Anthony Fauci has applauded Biden’s decision calling it “prudent” and necessary in a round of television interviews Monday.

“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 also expanded on the CDC requirement and directed that federal agencies require international travellers to quarantine upon arrival in the U.S. and obtain another negative test to slow the spread of the virus. Those requirements also go into effect Tuesday.

The State Department said in a statement that U.S. citizens should reconsider non-essential travel abroad, noting that access to testing in some nations remains difficult. The agency also cautioned Americans to consider ahead of international travel how they’d pay for health care and additional lodging costs if they became infected or hospitalized while travelling.

Here is a list of countries part of the border-free Schengen zone affected by the ban:

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.