The U.S State Department and Centers for Disease Control and Prevention on Saturday advised against travel to eight southern African countries after the White House announced travel restrictions in response to a new COVID-19 variant.

Omicron, dubbed a “variant of concern” by the World Health Organization, is potentially more contagious than previous variants of the disease. It could take weeks for scientists to fully understand the variant’s mutations and whether existing vaccines and treatments are effective against it.

On Saturday Vice President Kamala Harris urged Americans to get their shots.

“I cannot stress enough: One, if you have not had the booster shot, get the booster shot. I cannot stress enough the importance of getting vaccinated for those who have not been vaccinated.”

So far, the Omicron variant has not been detected in the United States. But Dr. Anthony Fauci, the nation’s top infectious disease specialist, said on NBC’s “Weekend TODAY” show, “I would not be surprised if it is.”

The variant was first discovered in South Africa and had also since been detected in Germany, Italy, Belgium, Botswana, Israel and Hong Kong.

Britain detected two cases of the new variant on Saturday. In response, Prime minister Boris Johnson laid out measures that included stricter testing rules for people arriving in the country.

“We’re not going to stop people travelling, but we will require anyone who enters the UK to take a PCR test by the end of the second day after their arrival and to self isolate until they have a negative result.”

Israel said it would ban the entry of all foreigners into the country and reintroduce counter-terrorism phone-tracking technology to contain the spread of the variant.

Dutch health authorities said they had detected 61 COVID-19 cases among people who flew from South Africa on Friday and believe at least some infections are the omicron variant.

Although epidemiologists say travel curbs may be too late to stop Omicron from circulating globally, many countries around the world - including the United States, Brazil, Canada and European Union nations - announced travel bans or restrictions on southern Africa on Friday.

A U.S. official told Reuters Friday the Biden administration could also add other countries to the travel curb list if the variant spreads.