Updated July 27, 2021 at 3:09 PM ET

The Centers for Disease Control and Prevention revised its guidance on wearing masks Tuesday. In a reversal of its earlier position, the agency is now recommending that some fully vaccinated people wear masks indoors if they live in areas with significant or high spread.

Currently, much of the country falls into that category — with the exception of the Northeast and parts of the Upper Midwest. The CDC provides this link if you want to see the area of spread in the county where you live.

"This was not a decision that was taken lightly," said Dr. Rochelle Walensky, the CDC's director, acknowledging that people are "tired and frustrated."

But Walensky pointed to new data showing that while vaccinated people still account for a small amount of risk, in rare cases they can get infected and spread the virus to others.

"The delta variant is showing every day its willingness to outsmart us," Walensky said.

" When we examine the rare breakthrough infections and we look at the amount of virus in those people, it is pretty similar to the amount of virus in unvaccinated people," she said. And, it's possible that because of this higher viral load, people with breakthrough cases of delta may be able to spread it.

"Unlike the alpha variant that we had back in May where we didn't believe that if you were vaccinated, you could transmit further, this is different now with a delta variant," she explained.

The CDC move comes as the highly transmissible delta variant fuels a surge in cases around the country, and multiple cities have reinstated indoor mask mandates, including in Los Angeles County and St. Louis. For weeks, many public health experts had 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.

"We have places that are now reporting over 300 cases per 100,000, so an extraordinary amount of viral transmission," Walensky said. And she again urged people who have not been vaccinated to get their shots.

"The highest spread of cases and severe outcomes is happening in places with low vaccination rates and among unvaccinated people," she said. And "the associated illness, suffering and death could have been avoided."

In addition, the agency recommends that all teachers, staff and students of K-12 schools wear masks, even if they are vaccinated. As the delta variant spreads, particularly in areas where vaccination rates are low, kids remain unprotected against the virus. The vaccines are not authorized for children under 12, and many teenagers have yet to get vaccinated.

The nation's leading group of pediatricians also recommends mandatory masking in schools for all students (over 2 years old), staff and teachers regardless of vaccination status.

In May, the agency signaled 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 administered on the day the policy was announced. "You can do things you stopped doing because of the pandemic," 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. haven't been 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, said 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 said. "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, said 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 said.

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