CDC expected to revise coronavirus mask guidance as delta spreads

The Centers for Disease Control and Prevention (CDC) is expected to announce updates to its indoor COVID-19 mask guidelines on Tuesday (July 27), as the highly transmissible coronavirus delta variant rapidly spreads across the country, according to recent news reports.

The agency is expected to recommend that people who live in areas with high or substantial COVID-19 transmission start wearing masks again, even if they are fully vaccinated, a source told CNN. That means more than half of U.S. counties should resume mask wearing; currently, about 46% of U.S. counties have "high" transmission of COVID-19 and 17% have "substantial" transmission, according to CDC data. 

This expected announcement comes as COVID-19 cases continue to rise across the country, largely driven by the delta variant, which is especially ravaging areas that have low vaccination rates. But "breakthrough infections" (which tend to be more mild) are also being reported among those who are fully vaccinated, according to The New York Times. What's more federal health officials say there’s a chance that fully vaccinated people could transmit COVID-19 to others, but they believe that the majority of transmission occur among those who are unvaccinated, according to CNBC. 

Related: Delta variant: Your questions answered

The CDC had last updated its mask guidance in May to say that fully vaccinated people can go to most indoor and outdoor areas without masks; and unvaccinated people can go outdoors without masks if they're not in crowded areas, Live Science previously reported.

But now, the U.S. is "going in the wrong direction," Dr. Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases said on CNN's "State of the Union" on Sunday (July 25). "This is an issue predominantly among the unvaccinated, which is the reason why we're out there, practically pleading with the unvaccinated people to go out and get vaccinated," he said.

The delta variant is currently the dominant coronavirus variant in the country and it is thought to be 60% more transmissible than the previous dominant variant (the alpha variant.) It's still not clear exactly why the delta variant is so infectious, but a small study (that hasn't yet been peer-reviewed) suggests that people infected with the delta variant may be carrying more than a thousand times more virus particles and may test positive two days earlier than those infected with the original virus, Live Science previously reported.

Experts, including Fauci, have repeatedly said that the best defense against the delta variant would be to get vaccinated. Yet, only 49.1% of the U.S. population is fully vaccinated against COVID-19, and only 56.8% have received at least one dose, according to CDC data. 

Some local governments have already taken their own initiative. Los Angeles County, the country's most populous county, already reinstated a mask mandate last week that requires everyone — both vaccinated and not vaccinated — to mask up indoors, CNN reported.

Originally published on Live Science.