THE DELTA VARIANT of coronavirus.

The supercontagious Delta variant of the coronavirus is responsible for about 1 in 5 COVID-19 cases in the United States, and its prevalence has doubled in the last two weeks, heath officials said Tuesday.

First identified in India, Delta is one of several “variants of concern,” as designated by the Centers for Disease Control and Prevention and the World Health Organization. It has spread rapidly through India and Britain.

Its appearance in the United States is not surprising. And with vaccinations ticking up and COVID-19 case numbers falling, it’s unclear how much of a problem Delta will cause here. Still, its swift rise has prompted concerns that it might jeopardize the nation’s progress in beating back the pandemic. “The Delta variant is currently the greatest threat in the U.S. to our attempt to eliminate COVID-19,” Dr. Anthony Fauci, the nation’s leading infectious disease expert, said at the briefing. The good news, he said, is that the vaccines authorized in the United States work against the variant. “We have the tools,” he said. “So let’s use them and crush the outbreak.”