July 27, 2021 -- The CDC is once again is recommending some Americans wear masks indoors. The agency called today for masks in K-12 school settings and in areas of the U.S. experiencing high or substantial SARS-CoV-2 transmission, even for the fully vaccinated against COVID-19.

The move reverses a controversial announcement the agency made in May 2021 that stated fully vaccinated Americans could skip wearing a mask in most settings.

Unlike the increasing vaccination rates and decreasing case numbers reported in May, however, some regions of the U.S. are now reporting large jumps in COVID-19 case numbers. And the Delta variant, as well as new evidence for transmission from breakthrough cases, are largely driving these changes.

"Today we have new science related to the Delta variant that requires us to update the guidance on what you can do when you are fully vaccinated," CDC Director Rochelle Walensky, MD, said during a media briefing on Tuesday.

New evidence has emerged on breakthrough case transmission risk, for example.

"Information on the Delta variant from several states and other countries indicates that in rare cases, some people infected with the Delta variant after vaccination may be contagious and spread virus to others," Walensky said, adding that the viral loads appear to be about the same in vaccinated and unvaccinated individuals.

"This new science is worrisome," she said.

Even though unvaccinated people represent most cases of transmission, "we thought it was important for [vaccinated] people to understand they have the potential to transmit the virus to others,” Walensky said.

As a result, in addition to continuing to strongly encourage everyone to get vaccinated, the CDC recommends fully vaccinated people wear masks in public, indoor settings to help prevent spread of the Delta variant in areas with substantial and high and transmission.

"This includes schools,” Walensky said.

The CDC is now recommending universal indoor masking for all teachers, staff, students and visitors to K-12 schools, regardless of vaccination status. Their goal is to optimize safety and allow children to return to full-time in-person learning in the fall.

The CDC tracks substantial and high transmission rates through the agency's COVID Data Tracker site. Substantial transmission means between 50 and 100 cases per 100,000 people reported over 7 days and high means more than 100 cases per 100,000 people.

The B.1.617.2 or Delta variant is believed responsible for COVID-19 cases increasing more than 300% nationally from June 19 to July 23, 2021, for example.

"The highest spread of cases and [most] severe outcomes are happening in places with low vaccination rates and among unvaccinated people. With the Delta variant, vaccinating more Americans now is more urgent than ever,” Walensky said. "This moment, and the associated suffering, illness and death, could have been avoided with higher vaccination coverage in this country."