The highly transmissible Delta variant of the Covid-19 virus is spreading so rapidly in the U.S. that it could become the dominant strain in the next two to three weeks, researchers said, adding urgency to the nationwide vaccination drive. The Delta strain, which first emerged in India in late 2020 and is also known as B.1.617.2, will probably make up 50% of Covid-19 infections in the U.S. by early to mid-July, said William Lee, vice president of science at population genomics company Helix and an author of the new analysis.
The U.S. Centers for Disease Control and Prevention released data on Tuesday showing that the prevalence of the Delta strain more than doubled over the two weeks ended June 19 to 20.6% of positive Covid-19 test samples genetically sequenced, compared with 9.9% in the week ended June 5. “The Delta variant is currently the greatest threat in the U.S. to our attempt to eliminate Covid-19,” Anthony Fauci, President Biden’s chief medical adviser, said at a briefing on Tuesday.
“Based on CDC data and the experience in the United Kingdom, we would also anticipate that the Delta variant is likely to become the dominant variant in the United States over the next month,” she said in a statement.
Delta now accounts for nearly half of Covid-19 cases in some Midwest and mountain states, she said at the briefing.
Delta is competing to displace the current dominant strain in the U.S. with another variant, Gamma, which is also causing a growing number of Covid-19 cases, though it is spreading more slowly, the study found.
Public-health officials say the Delta variant is particularly risky to people who are unvaccinated. Delta is about 50% more transmissible than the current dominant U.S. strain, called Alpha, they say, which itself is 50% more transmissible than the most commonly circulating strain last year.
The Delta variant has also been associated with more severe disease, Dr. Fauci said. Full vaccination against Covid-19 appears to offer significant protection against the Delta variant, according to data from Public Health England.