US President Joe Biden will impose a ban on most non-US citizens entering the country who have recently been in South Africa starting Saturday in a bid to contain the spread of a new variant of COVID-19, a senior US public health official told Reuters.

Biden on Monday is also reimposing an entry ban on nearly all non-US travelers who have been in Brazil, the United Kingdom, Ireland and 26 countries in Europe that allow travel across open borders.

“We are adding South Africa to the restricted list because of the concerning variant present that has already spread beyond South Africa,” said Dr. Anne Schuchat, the CDC’s principal deputy director, in an interview Sunday.

She added the agency was “putting in place this suite of measures to protect Americans and also to reduce the risk of these variants spreading and worsening the current pandemic.”

Then-President Donald Trump directed on January 18 those restrictions on Brazil and Europe be lifted effective Tuesday but Biden’s proclamation will rescind that decision.

Biden, who took office on Wednesday, is taking an aggressive approach to combating the spread of the virus after Trump rejected mandates sought by US health agencies.

Some health officials are concerned that current vaccines may not be effective against the South Africa variant, which also raises the prospect of re-infection.

The South African variant, also known as the 501Y.V2 variant, is 50% more infectious and has been detected in at least 20 countries. CDC officials told Reuters they would be open to adding additional countries to the list if needed.

The South African variant has not yet been found in the United States but at least 20 US states have detected a UK variant known as B.1.1.7. Current vaccines appear effective against the UK mutations.

The South African embassy in Washington did not immediately respond to a request for comment.