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Half of new US COVID-19 cases now tied to delta variant, CDC says

People lined up to get a COVID-19 test in Miami.Marta Lavandier/Associated Press

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.

The proportions of the variant across the country vary, ranging from 30% in the Pacific Northwest to greater than 80% in the Midwestern region of the country.

The CDC’s Fulce stressed that variant proportions are difficult to predict due to reporting delays, the presence of multiple variants, and changing incidence. The agency is looking at approaches to improve genomic surveillance in order to make more reliable predictions, she added.

Delta, which first emerged in India, is spreading around the globe as governments race to inoculate people. The mutation has already forced some countries to delay or rethink plans to loosen curbs on businesses, activity and travel.