CNN  — 

The US Centers for Disease Control and Prevention recommended on Tuesday that localities encourage all teachers, staff, students, and visitors in schools to wear masks indoors, regardless of vaccination status.

“In areas with substantial and high transmission, CDC recommends fully vaccinated people wear masks in public indoor settings to help prevent the spread of the Delta variants and protect others – this includes school,” CDC Director Dr. Rochelle Walensky told a media briefing on Tuesday.

“CDC recommends that everyone in K through 12 schools wear a mask indoors, including teachers, staff, students and visitors, regardless of vaccination status. Children should return to full-time in-person learning in the fall with proper prevention strategies in place,” she said. “Finally, CDC recommends community leaders encourage vaccination and universal masking to prevent further outbreaks in areas of substantial and high transmission with the Delta various. Vaccinating more Americans now is more urgent than ever.”

Previously, the CDC updated its Covid-19 guidance for schools earlier this month, noting that fully vaccinated people did not have to wear masks, and then about a week later the American Academy of Pediatrics issued stricter guidance recommending that everyone older than 2 wear a mask in schools, regardless of their vaccination status.

Now, the agency is updating its mask guidance overall, recommending that fully vaccinated people wear masks indoors when in areas with “substantial” and “high” transmission of Covid-19, which includes more than half of all US counties.

Nearly half – 46% – of US counties currently have high transmission and 17% have “substantial” transmission, according to data from the CDC, as of Tuesday morning.

“When we released our school guidance on July 9, we had less Delta variant in this country, we had fewer cases in this country, and importantly, we were really hopeful that we would have more people vaccinated, especially in the demographic between 12 to 17 years old,” Walensky said.

“Next week, we have many school systems that are starting around the country, and I think we all agree that children 11 or less are not going to be able to be vaccinated,” she said. “With only 30% of kids between 12 and 17 fully vaccinated now, more cases in this country, and a real effort to try and make sure that our kids can safely get back to full in person learning in the fall, we’re recommending that everybody wear masks right now.”

The nation’s three largest school districts – New York City, Los Angeles and Chicago – already require masks in schools. Among the top 12 districts in the country, all are requiring masks except those in Florida and Texas, where the governors have banned mask mandates in schools, prior to Tuesday’s updated CDC guidance.

For instance, in Texas, “earlier this summer, Governor Abbott issued an executive order banning mask mandates on public school campuses so HISD cannot require them but those who want to wear mask may certainly do so,” Dennis Spellman, spokesperson for Houston Independent School District, told CNN in an email on Tuesday.

Some other schools, such as Little Rock School District in Arkansas, told CNN on Tuesday that they are reviewing the new CDC recommendations.

Before the updated CDC guidance, at least 14 of the 30 largest US school districts overall are making masks optional for students in school, while another 13 are requiring masks. As of last week, three districts were undecided. School districts have indicated they may change their guidance according to local public health conditions even after the school year starts.

Caitlin Rivers, a senior scholar at Johns Hopkins Bloomberg School of Public Health’s Center for Health Security, told CNN in an email on Tuesday that she thinks the updated CDC guidance will be enough to encourage some schools to issue mask mandates.

“School districts look to CDC for guidance so having clear recommendations will make it easier for them to set masking policies,” Rivers wrote in the email.

“I encourage indoor masking for children too young to be vaccinated,” she added. “The delta variant spreads very easily, and classrooms are high risk environments without protective measures. Children are at much lower risk of severe illness than adults, but they can and do get infected and transmit the virus to others.”

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Overall, it’s “reassuring” that the CDC is updating its guidance given that the Delta variant has been shown to be more contagious, Dr. Vivek Cherian, an internal medicine physician affiliated with the University of Maryland Medical System, told CNN in an email on Tuesday. He called the updated guidance for schools “the best and safest course of action” to protect children at this time.

“On a personal note, we are in the process of moving to Chicago and are basing our decision on school selection based off of their approach to mask mandate because it is that critical to offer our children the greatest advantage of staying safe,” Cherian said about his own family.

“There is no question that the updated recommendations will encourage some schools (if not most) to issue or reissue mask mandates. Many schools are implementing guidance based on the CDC recommendations in general,” Cherian wrote. “I would consider it reckless on the part of schools to not move forward in updating mask mandates given that the Delta variant is now the predominant strain in the country, and given that a large population of the K-12 population is still unvaccinated.”

CNN’s Elizabeth Stuart contributed to this report.