(NEXSTAR) – Dr. Anthony Fauci warned Tuesday that new COVID-19 cases are far outpacing the number of vaccinated Americans needed to slow the delta variant and help prevent a potentially more deadly mutation of the virus.

When asked by “Morning Joe” co-host Mika Brzenzinski if the delta variant might lead to a “monster” variant, the chief White House infectious disease expert agreed there was a risk, so long as the virus continues to spread aggressively.

“One of the reasons why we want to make sure that we get as many people vaccinated as we possibly can is that viruses will not mutate if they don’t have the opportunity to spread and replicate.”

Fauci added that the current vaccine drive is being done to prevent future variants.

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“You don’t want to see more variants come in because then it would in many respects it would negate some of the very positive protections from the vaccines.”

COVID-19 deaths and cases in the U.S. have climbed back to levels not seen since last winter, erasing months of progress and potentially bolstering President Joe Biden’s argument for his sweeping new vaccination requirements.

“In a country of our size, you can’t be hanging around and having 100,000 infections a day. You’ve got to get well below 10,000 before you start feeling comfortable,” Fauci told Axios last week.

The cases — driven by the delta variant combined with resistance among some Americans to getting the vaccine — are concentrated mostly in the South.

While one-time hot spots like Florida and Louisiana are improving, infection rates are soaring in Kentucky, Georgia, and Tennessee, fueled by children now back in school, loose mask restrictions, and low vaccination levels.

The biggest surge over the summer occurred in states that had low vaccination rates, particularly in the South, where many people rely on air conditioning and breathe recirculated air, said Linsey Marr, a professor of civil and environmental engineering at Virginia Tech. She said states farther north could see upticks as the onset of cold weather sends people indoors.

Vaccination rates are not as low in some Northern states, but “there’s still a lot of unvaccinated people out there. Delta is going to find them,” Marr said.

The U.S. is averaging over 1,800 COVID-19 deaths and 170,000 new cases per day, the highest levels respectively since early March and late January. And both figures have been on the rise over the past two weeks.

The country is still well below the terrifying peaks reached in January when it was averaging about 3,400 deaths and a quarter-million cases per day.

The U.S. is dispensing about 900,000 vaccinations per day, down from a high of 3.4 million a day in mid-April. On Friday, a Food and Drug Administration advisory panel will meet to discuss whether the U.S. should begin giving booster shots of the Pfizer vaccine.

The Associated Press contributed to this report.