The highly contagious variant first identified in the U.K. likely accounts for up to 30% of Covid-19 infections in the United States, White House Chief Medical Advisor Dr. Anthony Fauci said Friday.

The variant, called B.1.1.7, has been reported in at least 94 countries and detected in 50 jurisdictions in the U.S., Fauci said during a White House news briefing on the pandemic, adding that the numbers are likely growing.

The U.K. first identified the B.1.1.7 strain, which appears to spread more easily and quickly than other variants, last fall. It has since spread across the world, including the U.S., Fauci said. U.S. researchers have identified 5,567 cases through genetic sequencing as of Thursday, according to the Centers for Disease Control and Prevention. U.S. health officials say the variant could become the dominant strain in the U.S. by the end of this month or in early April.

New variants are especially a concern for public health officials as they could become more resistant to antibody treatments and vaccines. Top health officials, including Fauci, have urged Americans to get vaccinated as quickly as possible, saying the virus can’t mutate if it can’t infect hosts and replicate.

Public health officials and Americans can counter the variant by doing two things, Fauci said. “Get as many people vaccinated as quickly and as expeditiously as possible with a vaccine that we know works against this variant, and finally to implement the public health measures that we talked about all the time … masking, physical distancing and avoiding congregate settings, particularly indoors.”

A recent study published in the British Medical Journal found the variant is associated with a 64% higher risk of dying from Covid-19 than earlier strains. Researchers at the University of Exeter and the University of Bristol analyzed data from more than 100,000 patients in the U.K. between Oct. 1 and Jan. 28.

“We’re at a position right now where we have a plateauing at around 53,000 cases per day,” Fauci said. “The concern is that throughout the country there are a number of states, cities, regions that are pulling back on some of the mitigation methods that we’ve been talking about: the withdrawal of mask mandates, the pulling back to essentially non-public health measures being implemented.”

Fauci’s comments on B.1.1.7 come a day after he squared off with Republican Sen. Rand Paul over masks at a hearing.

Paul maintained that people shouldn’t have to wear masks after getting vaccinated because there is “virtually 0% chance” they are going to get Covid-19. “Isn’t it just theater?” the Kentucky junior senator, an ophthalmologist, asked during a Senate Health, Education, Labor and Pensions Committee hearing.

In response, Fauci said the emergence of new, highly contagious variants poses a threat to people who have antibodies. ″Can I just state for the record that masks are not theater,” Fauci said. “I totally disagree with you.”