The delta variant now accounts for more than 83 percent of new Covid-19 cases, Dr. Rochelle Walensky, director of the Centers for Disease Control and Prevention, said Tuesday.
Walensky spoke during a hearing of the Senate Committee on Health, Education, Labor and Pensions.
As of July 17, the latest date for which data was available, the delta variant accounted for 83.2 percent of new Covid-19 cases that had been genetically sequenced in the country. The CDC is expected to publish these latest statistics online Tuesday afternoon.
The rise in the delta variant coincides with a large increase in Covid-19 cases, hospitalizations and deaths nationwide in the U.S., the CDC said last week.
The surge in cases involving the "hypertransmissible" variant was not unexpected. Such infections had been predicted to double every few weeks.
Just one month ago, on June 19, the delta variant accounted for just over 30 percent of new cases. On July 3, it crossed the 50 percent threshold to become the dominant variant in the country.
Studies have shown that the Covid-19 vaccines are effective against multiple variants, including the delta variant.
But the new data comes as vaccinations in the U.S. have slowed. As of Tuesday, less than half of the country's population — 48.6 percent — was fully vaccinated.
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Those who remain unvaccinated are most at risk for infection. That includes children under age 12, for whom no vaccine is available. There is no evidence that the variant is hitting children harder, other than the fact that they are vulnerable because they cannot yet be vaccinated.
"We are going to see pediatric Covid," said Dr. Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine in Houston. "I think part of a steep acceleration of the delta variant is going to sweep kids along with it."
Emergency use authorization of vaccines for children may not be available until midwinter, a Food and Drug Administration official recently told NBC News.