White House senior medical advisor Dr. Anthony Fauci said Wednesday the Delta variant now accounts for about 20% of newly diagnosed cases in the U.S. and will become the dominant Covid strain in the nation in just a few weeks.

“It just exploded in the UK. It has gone from a minor variety to now over 90% of isolates in the UK,” Fauci said on NBC’s TODAY show. According to Fauci, the variant has a doubling time of about two weeks and currently makes up 20% of isolates in the US that are newly diagnosed infections. “So you’d expect just the doubling time to be pretty dominant in a few weeks to a month or so, that’s the sobering news.”

The variant first originated in India and quickly spread across the globe. World Health Organization officials said Monday that deltas have been found in at least 92 countries. Dr. Mike Ryan, head of WHO emergency programs, said the variant is the fastest and strongest strain of coronavirus to date and will “pick up” the most vulnerable people, especially in places with low Covid-19 vaccination rates.

At a Covid briefing at the White House on Tuesday, Fauci named the variant the greatest threat to US efforts to contain the pandemic. Fauci said current vaccines in the US are effective at protecting most people from the Delta and other variants. CDC director Dr. Rochelle Walensky said the Delta variant was “hyper” communicable and urged people to get vaccinated.

Studies suggest that it is about 60% more transmissible than the alpha variant that appeared in the UK last fall and was already more contagious than the original strain that emerged from Wuhan, China, in late 2019.

Dr. Barbara Taylor, associate professor of infectious diseases at the University of Texas Health Science Center at San Antonio, said she was concerned the US is likely to see much of the Delta variant this summer and fall.

“We’re all pretty concerned about it,” she said. “It looks like it is more transmissible, it looks like it can re-infect people who have already had Covid. It looks like it has caused more serious illnesses and increased mortality in some situations. “

She said it is of particular concern for people who have not yet been vaccinated and adds “much urgency” to vaccination efforts both in the US and around the world.

“If you think of Texas, where we have had huge vaccination differences between urban and rural areas, and problems with vaccine access and uptake, it means much of our community is at risk from a variant that seems to be more easily transmissible is. ” and heavy, ”she said.

President Joe Biden said last week the variant was particularly dangerous for younger people who have the lowest vaccination rates. As of Friday, the variant accounted for 10% of all new cases in the US, up from 6% the week before.

“It is a variant that is more easily transmissible, potentially more deadly and particularly dangerous for young people,” he said on Friday.

The U.S. has vaccinated 177.6 million people with at least one dose, which is roughly 53% of the population, according to the Centers for Disease Control and Prevention. Only 13.6% of 18- to 24-year-olds in the United States received at least one dose of vaccine, compared with 26% of people ages 50 to 64, the data shows.