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U.S President Joe Biden willban entry for most non-U.S citizens who have recently been in South Africa in a bid to contain the spread of a new COVID-19 variant.

That's according to a senior U.S. public health official, who told Reuters that the ban will start Saturday.

In a CBS interview on Sunday, Biden's top COVID-19 medical adviser Anthony Fauci said he was concerned that current vaccines may not be effective against the South African variant, which raises the prospect of re-infection.

FAUCI: "A little bit more concerning with the South African isolate, namely the mutant that is now prevalent in South Africa, particularly its negative impact on some of the monoclonal antibodies that have been given for treatment, that it can, in some respects, knocks out their efficacy. It looks like it does diminish more so the efficacy of the vaccine. But we're still within that cushion level of the vaccines being efficacious against these mutants."

The South African variant has not yet been detected in the U.S. It's 50% more infectious and has been found in at least 20 other countries.

But the U.S. has detected a more contagious UK variant, though current vaccines appear effective against it.

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

Former President Donald Trump had directed the restrictions on Brazil and Europe be lifted effective Tuesday but Biden's proclamation will rescind that decision.

Meanwhile, new orders from the U.S. Centers for Disease Control will take effect this week...

In country, masks will be required on all public transportation and in ride-sharing vehicles.

CDC officials may require testing for domestic travel, as is required for international travelers entering the U.S.,but have made no decisions.