The Centers for Disease Control and Prevention is expected to revise its guidance on wearing masks later today. In a reversal of its earlier position, the agency is expected to recommend some fully vaccinated people wear masks indoors, in areas where the virus is circulating widely.

This comes as the highly transmissible delta variant fuels a surge in cases around the country and multiple cities have reinstated indoor mask mandates including Los Angeles County and St. Louis, Mo. For weeks, many public health experts have been nudging the agency to change its policy, arguing that fully vaccinated people should wear masks in indoor settings, especially in areas where transmission of the virus is high.

The agency is also expected to update its masking policy in schools, given the rise in cases around the country. As the delta variant spreads, particularly in areas where vaccination rates remain low, kids remain unprotected against the virus. The vaccines are not authorized for children under 12 and many teenagers have yet to get vaccinated.

Currently the CDC guidance is that fully vaccinated students and teachers don't need masks. But the nation's leading group of pediatricians recommends mandatory maskingin schools for all students (over 2 years old), staff and teachers, regardless of vaccination status.

Back in May, the agency signaled that it was safe for fully vaccinated people to stop masking in most settings. At that time cases were dropping significantly and the vaccination campaign was in full swing. Nearly 2 million shots were administeredon the day the policy was announced. "You can do things you stopped doing because of the pandemic," the CDC's director, Dr. Rochelle Walensky, said at the time.

The hope was that dropping the mask mandate would encourage more people to get vaccinated. Back in April when the outdoor mask mandate was lifted, President Biden said that for those who haven't been vaccinated, or feel they don't need to be, "this is another great reason to go get vaccinated now."

But, three months later about 30% of adults in the U.S. have yet to be vaccinated. And polls suggest that up to 80%of unvaccinated adults are unlikely to change their minds.

The decision to lift the mask mandate in the spring wasn't well thought out, says Dr. Zeke Emanuel, a health policy researcher at the University of Pennsylvania, and the agency has been delaying taking action as the delta variant spreads.

"I think the CDC position on masks has been behind the eight ball almost every step of the way," he says. "I don't think that they've been on top of the mask issue."

The pressure on the CDC to amend its masking policy in schools has been growing. School administrators don't have the resources to monitor who has been vaccinated or not, says Dr. Judy Guzman-Cottrill, a pediatric infectious disease expert at Oregon Health & Science University.

And, absent universal masking, kids will be left unprotected — or may be tempted to take off their masks. "I worry that some students will be singled out for wearing masks at school and this can lead to bullying and peer pressure to unmask even when it's not safe to do so," Guzman-Cottrill says.

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