Medical experts say they're keeping a close watch on the Midwest and Northeast as COVID-19 cases surge.

Those experts say the virus' U.K. variant threw the U.S.' efforts to get the pandemic under control. And now, they say it's a big part of why the country is seeing a rise in COVID-19 cases.

When looking at the case count from the past 30 days, compared to the previous 30, those that have seen the biggest jump are largely in the Midwest and Northeast.

Just five states account for about 43% of all new cases over the past week -- New York, New Jersey, Florida, Pennsylvania and Michigan.

Doctors are also noting that the face of COVID-19 right now is a lot younger.

"Hospitals are seeing more and more younger adults, those in their 30s and 40s," says CDC Director Dr. Rochelle Walensky.

One expert thinks states that are seeing surges should get more vaccines.

"We've got to be able to do this when there are surges to try to help states out," says Dr. Ashish Jha, dean of the Brown University School of Public Health.

But another worries that won't be enough.

"We just aren't going to be able to get enough people vaccinated quickly enough to keep the rest of the country from experiencing much of what we're seeing in the upper Midwest and Northeast," says epidemiologist Michael Osterholm.

More than 8,300 additional New Yorkers tested positive for COVID-19 Thursday. The positivity rate is now about 3.2%.