The delta variant is set to become the predominant Covid strain in the U.S., boosting concerns it could lead to a surge in cases in under-vaccinated hot spots.

The highly transmissible variant made up 30% of positive samples sequenced in the U.S. for the two-week period ending June 19. The variant is predicted to increase to 52% in the two weeks ending July 3, according to Jade Fulce, a spokesperson at the U.S. Centers for Disease Control and Prevention.

In June, the agency classified the B.1.617.2 strain, first identified in India, as a variant of concern. On July 1, the Biden administration announced the deployment of response teams to combat delta’s spread, and U.S. health officials have promised to boost testing and provide therapeutics.

“Without such measures, we will have surges throughout the coming year and we risk the emergence of a SARS-CoV-2 strain that could have higher rates of vaccine breakthrough and/or severe cases in vaccinated individuals,” said Samuel Scarpino, a co-founder of, an organization that tracks Covid cases and variants internationally.


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