The nation's top infectious disease expert cautioned on Sunday that the United States must move quickly to avoid a "dangerous" new wave of COVID-19 infections ahead of the Christmas season, with many Americans expecting to rejoin indoor gatherings for the first time since the pandemic broke out.

Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said it was not too late to prevent a significant worsening of infections in the weeks leading up to Christmas and New Year's if the public took action now on new measures to combat the virus, such as the approval of booster shots for adults and the recent opening of vaccinations to children aged five to 11.

Fauci said the U.S. still have around 60 million people who are eligible to be immunized but have not been. "This results in the dynamic of COVID-19 in the community, which is not only dangerous and makes people who are unvaccinated vulnerable, but it also spills over into the vaccinated people," Fauci said during CNN's State of the Union on Sunday.

Fauci's comments came as Europe continues to grapple with an alarming wave of new cases, pushing governments to impose stringent new limits in the hopes of averting hospital congestion and a repeat outbreak.

For the first time in weeks, new infections have begun to rise in the U.S. The seven-day average of new infections in the country soared by 36% this week compared to the previous week, and the number of daily new cases has once again topped 100,000.

The federal Transportation Security Administration (TSA) has forecast that the number of Americans traveling for Thanksgiving this year would be close to pre-pandemic levels, raising fears at a time when the Biden administration has struggled to get its vaccination message out.

"There are a lot of viruses going around... You can't ignore the evidence, and the data suggest that cases are beginning to rise, which is to be expected as we enter the winter season," Fauci said.

According to the U.S. Centers for Disease Control and Prevention, approximately 196 million Americans are completely immunized, accounting for nearly 60% of the total population.

Meanwhile, 83 million people, or 27% of the overall population, have yet to receive their first dose. In comparison to previous week, the virus' seven-day average has increased by 36%.

Fauci said the bottom line is: "Get vaccinated if you're not vaccinated, and boostered if you have been immunized."