The number of Covid-19 cases in the United States rose 10 percent this week as the highly contagious delta variant gained further ground, the Centers for Disease Control and Prevention said Thursday.

The country’s lagging vaccination rate coupled with the “hypertransmissible delta variant,” first detected in India, could account for the increase, CDC Director Dr. Rochelle Walensky said during a White House briefing.

The delta variant is about 60 percent more transmissible than current dominant strain in the U.S., the alpha variant. That variant, first detected in the United Kingdom, was more contagious than the original virus.

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The delta variant now accounts for a quarter of all new cases, and has been detected in all 50 states. Its rapid spread is sure to make it the dominant U.S. strain within the coming weeks, she said.

The seven-day average of new cases this week was about 12,600 cases, up 10 percent compared to last week’s average, Walensky said.

“We don’t want to alarm people,” she told NBC News after the briefing, “but we follow these numbers really, really carefully.”

While new cases are nowhere near the January peak of more than 247,000 Covid-19 cases per day, the shift makes some experts “a little nervous.”

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“I don’t want the trend line to put us in a position where it can really take off,” especially as fall approaches and people start congregating indoors, said Dr. Cameron Wolfe, an infectious diseases expert and an associate professor of medicine at the Duke University School of Medicine.

“I expect that we are going to see the number of cases around the country going up as the delta variant spreads,” said Dr. Richard Besser, president of the Robert Wood Johnson Foundation and a former acting director of the CDC. “The reason for that is that a significant number of people in America are still not vaccinated.”

As of Thursday, 57.4 percent of U.S. adults had been fully vaccinated, according to the CDC.

“It is clear that communities where people remain unvaccinated are communities that remain vulnerable,” Walensky said during the briefing.

For now, the uptick in Covid-19 cases does not appear to translate into more severe disease, at least on a national level. The seven-day average of Covid-19 hospitalizations across the U.S. has fallen by about 1 percent from last week, she said.

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Still, pockets of unvaccinated communities in the Southeast and the Midwest remain most vulnerable.

“As the delta variant continues to spread across the country, we expect to see increased transmission in these communities unless we can vaccinate more people now,” Walensky said.

The vaccines work well against all the variants in the U.S., including the delta variant, Dr. Anthony Fauci, director of the National Insitute of Allergy and Infectious Diseases, said during the briefing.

“If you are vaccinated, you have a very high degree of protection. If you are not, you should wear a mask, and you should think very seriously about getting vaccinated,” Fauci said. “The message is: Get vaccinated.”

The Biden administration had pushed for 70 percent of adults to have at least one dose by July 4, but acknowledged last week it would fall short of that goal. As of Thursday, 66.5 percent of adults had received at least one dose.

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