A coronavirus variant first spotted in India is poised to become the dominant one in the United States, where infectious disease modelers say it could cause a “resurgence” of Covid-19 later this year.

And it may already account for 1 in every 5 infections nationwide, experts say. The Delta variant, as it’s now called, has swept across the UK, all but replacing the Alpha variant first identified there late last year. “This is the most transmissible of all the variants that we’ve seen,” Dr. Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine, told CNN’s Ana Cabrera Monday. “We saw what happened in the UK, where it overtook the entire nation. So I’m worried that’s going to happen in the US,” he said.

Dr. Rochelle Walensky, director of the US Centers for Disease Control and Prevention, told CNN last week she anticipates Delta will become “the predominant variant in the months ahead.” And that could be a few weeks — not months — away, according to William Lee, vice president of science at Helix, a company whose Covid-19 tests have helped track a number of variants. In the two weeks leading up to June 5, CDC estimates that Delta was responsible for nearly 10% of US infections. And now, both Hotez and Lee say it likely accounts for roughly a fifth of cases. “It’s so transmissible that, unless your vaccination rates are high enough, you will still have outbreaks,” said Lee.