White House chief medical advisor Dr. Anthony Fauci said Wednesday the delta variant now accounts for roughly 20% of newly diagnosed cases in the U.S. and will become the dominant Covid strain in the nation in a matter of weeks.

“It just exploded in the U.K. It went from a minor variant to now more than 90% of the isolates in the U.K.,” Fauci said on NBC’s “TODAY” show. Fauci said the variant has a doubling time of about two weeks and currently accounts for 20% of the isolates in the U.S., which are newly diagnosed infections. “So you would expect, just the doubling time, you know, in several weeks to a month or so it’s going to be quite dominant, that’s the sobering news.”

The variant, which first emerged in India, has quickly swept across the globe. World Health Organization officials said Monday that delta has been found in at least 92 countries. Dr. Mike Ryan, head of WHO’s emergencies programs, said the variant is the fastest and fittest coronavirus strain yet, and it will “pick off” the most vulnerable people, especially in places with low Covid-19 vaccination rates.