• The delta variant first identified in India is now estimated to make up 83% of all sequenced Covid-19 cases in the U.S., the director of the CDC said Tuesday.
• "This is a dramatic increase from up from 50%, the week of July 3," CDC Director Dr. Rochelle Walensky said during a Senate hearing.
• Covid fatalities have risen by nearly 48% over the past week to an average of 239 per day, she said.

The delta variant first identified in India is now estimated to make up 83% of all sequenced Covid-19 cases in the U.S., a dramatic rise from 50% the week of July 3, the director of the Centers for Disease Control and Prevention said Tuesday.

CDC Director Dr. Rochelle Walensky, testifying at a Senate hearing, said nearly two-thirds of the counties in the U.S. have vaccinated less than 40% of their residents, "allowing for the emergence and rapid spread of the highly transmissible delta variant."

The surge in delta cases is leading to a rise in deaths. Covid fatalities have risen by nearly 48% over the past week to an average of 239 per day, she said. More than 34.1 million people in the U.S. have contracted the virus so far, killing more than 609,000, according to data compiled by Johns Hopkins University.

"Each death is tragic and even more heartbreaking when we know that the majority of these deaths could be prevented with a simple, safe, available vaccine," she said.

The variant is even more contagious than the alpha variant, which was first identified in the U.K. and was estimated by public health officials there to be between 43% and 90% more transmissible than the original Covid-19 strain. Discovered in October, delta has since spread to more than 100 countries, according to World Health Organization data.

"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," Dr. Anthony Fauci, the White House's chief medical advisor, said during the hearing.