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Due to emerging science about transmission in vaccinated people, the CDC now recommends all teachers and students wear masks in school, and that people living in COVID-19 hotspots, or areas of the country with large outbreaks, wear masks indoors, CDC officials said on Tuesday.

CDC Director Rochelle Walensky, MD, attributed this to new science surrounding the Delta variant, showing that "some vaccinated people infected with the Delta variant after vaccination may be contagious and spread the virus to others," she said at a briefing with reporters.

She cited evidence that in cohorts of COVID cases, the small number of fully vaccinated breakthrough cases had similar viral loads compared with unvaccinated cases. Walensky said that because of the potential for "forward transmission" in vaccinated people, CDC is updating its guidance to recommend that fully vaccinated people wear masks in areas with high or substantial transmission.

The agency also recommends "universal indoor masking" for everyone in K-12 schools, from teachers to students to staff and visitors, regardless of vaccination status.

"This moment ... and the associated illness, suffering, and death could have been avoided with higher vaccination coverage in this country," she said.

Both high and substantial transmission were defined using CDC's , where "substantial" transmission means 50 to 100 cases per 100,000 over a 7-day period, and "high" transmission means over 100 cases per 100,000, Walensky said. She added there were some U.S. counties with COVID rates of over 300 cases per 100,000. As of now, 46% of U.S. counties have "high transmission" and 17% have "substantial transmission."

"If you have a vaccinated individual in a place with substantial or high transmission" who are in contact with "a lot of people -- one in 20, one in 10 of those contacts -- could potentially lead to a breakthrough infection," she noted.

Community-level recommendations were outlined in further detail in an early edition of the

The CDC has been collecting PCR data on "tens of thousands of people," such as healthcare workers, essential workers, and workers in long-term care facilities, on a weekly or a biweekly basis to evaluate breakthrough infections, and will be reporting on those soon, Walensky added.

This represents the agency's third change in indoor mask guidance in the last 3 months. After dropping most mask recommendations for fully vaccinated people in May, the agency then said earlier this month that vaccinated teachers and students did not need to wear a mask indoors, though CDC still recommended indoor masking for students ages 2 and older who were not fully vaccinated.

Walensky also cited low uptake of vaccination among adolescents ages 12 to 17, with only 30% fully vaccinated.

"When we released our school guidance ... we had less Delta variant in this country ... and importantly, we were hopeful that we would have more people vaccinated," she said, when asked why the agency previously said vaccinated individuals could go unmasked in schools.

Walensky acknowledged that this guidance "is not a welcomed piece of news that masking is going to be a part of people's lives who've been vaccinated."

"This is not a decision that we ... have made lightly," she said. "This weighs heavily on me ... and with all of America."

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