The number of COVID-19 cases in the United States rose 10% 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" could account for the increase, CDC Director Dr. Rochelle Walensky said during a White House briefing.
The variant, which was first detected in India, 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.
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."
"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% 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.
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% from last week, she said.
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% 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.
This story first appeared on NBCNews.com.