The Centers for Disease Control and Prevention updated its masking guidance Tuesday, recommending that everyone, whether they are vaccinated or not, wear a mask indoors in places where the coronavirus is spreading widely. The change was met with relief from experts who said masking up again is essential to combat the highly contagious delta variant.

"We know masks work, and they work against every variant that this virus has produced," said Ali Mokdad, a professor of global health at the University of Washington. "If we use them, they will save lives, save livelihoods and prevent us from shutting down our economy."

The CDC's updated recommendations come as a devastating fourth wave of infection is sweeping the nation. Cases are rising in every state, with hospitalizations surging in states with low vaccination rates, such as Missouri, Arkansas, Mississippi and Florida.

While the majority of the outbreaks are happening among unvaccinated individuals, wearing a mask regardless of vaccination status can protect the entire community, Mokdad said. And while the CDC's guidance focused on areas where the virus is spreading widely, Mokdad urged people everywhere to take precautions.

"The whole country is on fire," he said. "Covid-19 is rising in every state. We're dealing with a stubborn, aggressive variant and we all need to be very careful."