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The rapidly spreading Delta variant now accounts for 83% of sequenced Covid-19 cases in the United States, the nation’s public health leaders said Tuesday, and in parts of the country with low vaccination rates, it may be responsible for up to 90%.

Rochelle Walensky, the director of the Centers for Disease Control and Prevention, told a congressional hearing the new 83% estimate represents “a dramatic increase” from 50% of sequenced cases that were tallied the week of July 3. In some parts of the country that percentage is even higher, particularly in areas with low vaccination rates, said Anthony Fauci, head of the National Institute of Allergy and Infectious Diseases.

As the variant has spread, Covid-19 deaths in the U.S. have risen, climbing by nearly 48% over the past week and now averaging 239 per day, she said.


“We know the majority of deaths could be prevented by vaccination,” Walensky said.

Vaccination has been uneven, with levels below half of eligible people in two-thirds of the country, Walensky said. She cited a five-month CDC study that found 99.5% of Covid-19 deaths are among people who are unvaccinated.


“The best way to prevent the spread of Covid-19 variants is to prevent the spread of disease. And vaccination is the most powerful tool we have,” she said. “We must continue to expand vaccine coverage by building trust and confidence in Covid-19 vaccines. And this is particularly important in communities of color, rural communities, and other population groups at risk.”

Pressed to explain why the U.S. has not pursued booster shots as Israel has done with third Pfizer doses, Walensky and Fauci both pushed back. Walensky said the CDC is sharing data with Israel and the U.K. “where they’re several weeks ahead of us in the variant,” while watching more than 20 cohorts of vaccine recipients for any signs that their protection against Covid-19 might be waning.

So far protection from recognizable disease continues to hit percentages in the low 90s, Fauci said.

“If you see it fall below that into the 80s or even, unfortunately, I hope it never happens, into the 70s, then, you know, you reach the point where the durability needs a boost,” he said. “Those studies are ongoing right now.”

Fauci blamed the variant’s high transmissibility for the surge in cases among unvaccinated people. First identified in India, Delta has swept through 90 countries around the world. But the three vaccines approved in the U.S. — mRNA vaccines from Moderna and Pfizer and the adenovirus-delivered vaccine from Johnson & Johnson — continue to be highly effective against Delta and other variants.

“We have the tools to end this epidemic,” Fauci told the Senate Committee on Health, Education, Labor and Pensions. “It’s up to us to use those tools to the maximum.”

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