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 on Monday.

Biden will also reimpose 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.

The announcement is expected to be made on Monday.

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

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

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.

Biden has made fighting the coronavirus pandemic a priority for his first 100 days in office. Last week, he asked every American to mask up for 100 days.

"Our new COVID-19 plan starts with mounting an aggressive, safe and effective vaccination campaign," Biden explained.

"We’ll move heaven and earth to get more people vaccinated - for free. For the past year, we couldn't rely on the federal government to act with the urgency and focus and coordination we needed, and we have seen the tragic cost of that failure."