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WASHINGTON – The delta variant of the novel coronavirus accounts for more than 80% of all genetically sequenced COVID-19 cases nationwide, Dr. Rochelle Walensky, head of the Centers for Disease Control and Prevention, told senators on Tuesday.

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The more contagious mutation of the virus accounted for about 50% of all sequenced cases in the week of July 3, marking a sharp increase in delta variant cases. Speaking Tuesday at a hearing before the U.S. Senate Committee on Health, Education, Labor and Pensions, Walensky urged unvaccinated Americans to get their shots to stymie the spread of the virus and other variants.

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“Areas with limited vaccine coverage are allowing for the emergence and rapid spread of the highly transmissible delta variant,” she said. “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.”

The delta variant was first detected in India and has since spread to more than 90 countries across the globe, Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said Tuesday.

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“The reason it’s so formidable is the fact that it has the capability of transmitting efficiently from human-to-human in an extraordinary manner well beyond any of the other variants that we’ve experienced up to now, which has led to its becoming the dominant variant in this country,” he told senators.

As delta variant cases continue to rise, officials noted a dramatic increase in the number of virus-related deaths reported across the U.S.

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“Over the last week, we have averaged 239 deaths per day, an increase of nearly 48% over the prior week,” Walensky said. “Each death is tragic, and even more heartbreaking when we know that the majority of these deaths can be prevented with a simple, safe, available vaccine.”

Across the U.S., about 56% of the population, or 186.3 million people, had received at least one vaccine dose as of Monday morning, according to the latest numbers available from the Centers for Disease Control and Prevention. Officials said 161.4 million people have so far been fully vaccinated, amounting to about 49% of the population.

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More than 34.1 million cases of COVID-19 have been reported in the U.S. since the pandemic began, resulting in over 609,000 deaths, according to numbers compiled by Johns Hopkins University.

“The overwhelming majority of deaths from COVID-19 are now occurring in unvaccinated people,” Walensky said Tuesday. “Vaccines are widely available across the country and this suffering and loss is simply and entirely preventable.”