Experts say a confluence of factors is likely driving the country's recent increases in infections, including the more than 100 million Americans who remain completely unvaccinated, cold weather and relaxed restrictions, the highly transmissible delta variant, and waning vaccine immunity.

According to health officials, the vast majority of infections and severe hospitalizations continue to be among the unvaccinated.

This September, federal data showed that unvaccinated individuals have a 5.8 times greater risk of testing positive for COVID-19, and a 14 times greater risk of dying from it, as compared to those fully vaccinated.

Across the country, more than 100 million Americans remain completely unvaccinated -- 81 million of whom are currently over the age of 5, and thus are eligible to be vaccinated.

In the U.S., federal data shows that the delta variant accounts for 99.9% of new coronavirus cases.

The delta variant spreads more easily among vaccinated people than prior versions of the virus, though vaccinated people are still far less likely to spread the virus compared to unvaccinated people.

Although the vast majority of infections and severe hospitalizations are among the unvaccinated, breakthrough positive COVID-19 cases among the vaccinated do appear to be on the rise, due to waning immunity, according to health officials.