The Centers for Disease Control and Prevention (CDC) recommended Tuesday that fully vaccinated people start wearing masks again indoors in areas with high COVID-19 transmission rates, effectively reversing its previous guidance issued in May due to the spread of the Delta variant.
Under the federal agency's updated guidance, fully vaccinated people are advised to wear masks in public indoor spaces in areas of high or substantial COVID transmission to "maximize protection from the Delta variant and prevent possibly spreading it to others."
The CDC defines areas of high transmission as an area with at least 100 new cases per 100K residents over the past week, or reporting a 10% or higher positivity rate. To be categorized as an area of substantial transmission, an area has to report 50 to 100 cases per 100K residents, or an 8% to 10% positivity rate.
The CDC says 1,495 counties in the United States fall into the high transmission category and other 548 counties qualify for the substantial tier. Those counties combined make up about two-thirds of the U.S. population, according to CNBC.
The CDC notes that while COVID vaccines are highly effective against severe disease and death from COVID variants, including Delta, fully vaccinated people can still become infected and may further spread the disease. When these infections occur among fully vaccinated people, however, they tend to be mild, the CDC says.
"This pandemic continues to pose a serious threat to the health of all Americans," CDC Director Dr. Rochelle Walensky told reporters on a call Tuesday, reported by CNBC. "We have new science related to the Delta variant that requires us to update the guidance regarding what you can do when you are fully vaccinated."
Walensky told reporters that new data shows that the highly transmissible variant behaves "uniquely different from past strains of the virus," with research showing that some vaccinated people infected with the Delta variant "may be contagious and spread the virus to others."
The updated guidance comes ahead of the fall season, when many employers are expected to return employees to offices for at least half of the work week. Health experts also forecast that the Delta variant could cause another surge in new cases as people return to more indoor activities as the weather cools down.
Back in May, the CDC recommended that fully vaccinated people could forgo masks in most settings due to the effectiveness of the vaccine. The CDC considers an individual to be fully vaccinated two weeks after their second dose of either vaccines developed by Pfizer
