Delta Is Dominant Variant in the U.S., C.D.C. Estimates

The highly contagious Delta variant of the coronavirus is now the dominant variant in the United States, accounting for 51.7 percent of infections, according to new estimates from the Centers for Disease Control and Prevention.

© Elijah Nouvelage/Agence France-Presse — Getty Images
Volunteers walked door to door last week, during an outreach campaign to inform residents of an upcoming coronavirus vaccination event in Birmingham, Ala.

As health officials had predicted, the Delta variant has rapidly overtaken Alpha, the variant that spread through the United States this spring. Alpha, first detected in Britain, now makes up just 28.7 percent of infections, according to the C.D.C.

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Still, overall, the average numbers of new virus cases and deaths across the country, as well as hospitalizations, are significantly down from the devastating peaks during previous national surges.

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Delta was first detected in India. Research suggests that most vaccines still provide good protection against it and remain highly effective at preventing hospitalizations and deaths.

In England, for instance, where the variant now causes almost all infections, case numbers have risen sharply in recent weeks, but hospitalization rates have increased more slowly and remain low. Next week, a final decision will be made about whether to lift most remaining restrictions in England, including mask rules, on July 19.

Studies suggest, however, that a single shot of a two-dose regimen provides only weak protection against Delta, and public health experts have been encouraging Americans to get fully vaccinated as soon as possible.

As of Wednesday, 67.2 percent of adults in the United States have had at least one vaccine dose, and 58.4 percent are fully vaccinated.

Still, vaccination coverage remains highly uneven, both in the United States and globally, and public health experts say Delta poses a serious threat to unvaccinated populations. On Tuesday, President Biden again urged Americans to get their shots, citing concerns about Delta. “It sounds corny, but it’s a patriotic thing to do,” he said.

Health experts say the Biden administration may need to take more aggressive action to encourage vaccination, including urging employers and schools to adopt vaccine mandates.

Despite the spread of the Delta variant, the country has been averaging fewer than 15,000 new coronavirus cases a day for nearly a month, the lowest levels since testing became widely available.

In recent days, however, the average number of new cases has started to trend slightly upward nationally, driven largely by localized outbreaks in places with low vaccination rates, including parts of Missouri, Arkansas and Nevada.

As the Delta variant has spread, the World Health Organization recently reiterated its longstanding guidance that everyone, vaccinated or not, should wear masks as a precaution. In the United States, however, the C.D.C. has not changed its advice that those who are fully vaccinated can skip masks in most situations.

On Tuesday, Jen Psaki, the White House press secretary, said that the Biden administration was unlikely to impose new national mitigation measures, even if cases rise.

“The states are going to have to make evaluations and local communities are going to have to make evaluations about what’s in their interests,” she said.

Mitch Smith and Sharon Otterman contributed reporting.