By Stephanie Stahl

PHILADELPHIA (CBS) — There are fears of another COVID-19 wave as the more contagious Delta variant exploded in the United States, doubling in just the past two weeks. It’s a special danger to people who aren’t vaccinated, and that impacts large segments of certain populations in the Philadelphia region.

Children who can’t be vaccinated are most at risk, along with young African Americans who have very low rates of vaccination. They’re prime targets for this more contagious variant that is spreading quickly.

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The lab at Penn Medicine is doing genome sequencing that tracks different strains of COVID-19. The Delta variant, first identified in India, now accounts for more than 20% of all new U.S. COVID cases. That’s up from just over 2% last month.

“The combo of a virus that spreads more rapidly and can make you more ill is what we have to worry about,” Dr. Anthony Fauci said.

The biggest concern is for people who aren’t fully vaccinated, which is about 55% of the country. That includes children under 12 who aren’t eligible. In Philadelphia, young African Americans between the ages of 20 to 44, only 27% have received the first dose.

“They’re very vulnerable,” Frederic D. Bushman with Penn Medicine said. “We’ve been sequencing viral gene variants in the Delaware Valley, in our area. We have seen the Delta variant. It’s around. It’s here.”

Bushman, the chair of microbiology at Penn, says the Delta variant isn’t widespread in the Philadelphia region yet, but it’s expected.

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“The virus variants are arising all over the world, and there’s so much travel between different places, that it’s inevitable,” Bushman said. “Every variant will have an opportunity to affect some new place.”

Full doses of the vaccine work well against the variants, but health officials aren’t sure how long that will last.

“This variant represents a set of mutations that could lead to future mutations that evade our vaccine,” Dr. Rochelle Walensky, director of the U.S. Centers for Disease Control and Prevention, said.

But for now, vaccines are the best weapon against the variants and preventing a resurgence.

“The Delta variant is currently the greatest threat in the U.S. to our attempt to eliminate COVID-19,” Fauci said.

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The Delta variant is causing outbreaks in Europe, Israel, and Australia. Scientists say it’s 60% more contagious than the other variants.

Stephanie Stahl