The delta coronavirus variant, which the CDC estimates is about 60 percent more transmissible, now accounts for 20 percent of all new COVID-19 cases in the U.S. — up 10 percentage points from about two weeks ago, according to the White House chief medical advisor.
The strain is on a similar track as the alpha variant, first identified in the U.K., which became the most common U.S. strain in April. Infections from the delta strain are doubling about every two weeks in the U.S., Anthony Fauci, MD, director of the National Institute of Allergy and Infectious Diseases, said during a June 22 White House news conference.
"Similar to the situation in the U.K., the delta variant is currently the greatest threat in the U.S. to our attempt to eliminate COVID-19," Dr. Fauci said.
Among those who are unvaccinated, delta is believed to trigger severe illness in more people than other variants, with recent projections from the COVID-19 Scenario Modeling Hub predicting the strain could prompt moderate surges through regions with a high proportion of unvaccinated people as early as July.
Research has shown COVID-19 vaccines are largely effective against the strain, Dr. Fauci added.
"The effectiveness of the vaccines, in this case, two weeks after the second dose of Pfizer-BioNTech was 88 percent effective against the delta and 93 percent effective against alpha when dealing with symptomatic disease," he said in reference to a recent study.
The CDC classified delta as a "variant of concern" June 14, when the strain accounted for about 10 percent of U.S. cases.