The Centers for Disease Control and Prevention recommended Tuesday that fully vaccinated people should wear masks again indoors in locations with high Covid-19 transmission rates. The agency also recommends that children wear masks in schools in the fall.

Federal health officials still believe that fully vaccinated individuals represent a very low level of transmission, according to those familiar with the matter. Still, some people vaccinated could carry higher amounts of the virus than previously thought and potentially pass it on to others, they said.

“As you’ve heard from me, this pandemic continues to pose a serious threat to the health of all Americans,” CDC director Rochelle Walensky told reporters on a call. “Today we have new science related to the Delta area that requires us to update the guidelines on what to do when you are fully vaccinated.”

The updated guidelines come before the fall season, when the highly contagious Delta variant is expected to lead to a further surge in new coronavirus cases and many large employers plan to bring workers back to the office. In mid-May, the CDC announced that fully vaccinated people would not need to wear masks in most environments, whether indoors or outdoors.

“In areas with significant and high transmission, the CDC recommends fully vaccinated individuals wear masks in public, indoors, to prevent the spread of the Delta variant and to protect others. This includes schools, ”said Walensky. The CDC recommends that all elementary schools wear masks indoors, “including teachers, staff, students, and visitors regardless of vaccination status.”

Walensky said new data shows the variant behaves “uniquely differently from previous strains of the virus,” suggesting that some vaccinated individuals infected with the Delta variant “can be contagious and transmit the virus to others.”

Continue reading: Americans will need masks indoors as the US is heading for a “dangerous fall” with a surge in Delta Covid cases

Health experts fear that Delta, already the dominant form of the disease in the US, hits states with low vaccination rates. These states are now being forced to reintroduce mask rules, capacity limits and other public health measures that they have largely withdrawn in recent months.

White House senior medical advisor Dr. Anthony Fauci said Sunday that the CDC was considering revising mask guidelines for vaccinated Americans, saying it was “in active consideration”.

“It’s a dynamic situation. It’s in the works, it’s developing like so many other areas of the pandemic, “Fauci, also director of the National Institute of Allergies and Infectious Diseases, told CNN. “You need to look at the data.”

The CDC guidelines are just a recommendation, leaving it up to state and local officials to reintroduce their masking rules for specific individuals. But even before the CDC’s expected guidelines on Tuesday, some regions reintroduced mask mandates and notices as Covid cases rose again.

Several California and Nevada counties are now advising all residents to wear masks in public indoor spaces, regardless of whether they are vaccinated or not. In Massachusetts, Provincetown officials advised everyone to return to wearing masks indoors after the July 4 celebrations resulted in an outbreak of new cases.

Experts say Covid prevention strategies remain critical to protecting people from the virus, especially in areas with medium to high transmission rates in the community.

Dr. Paul Offit, a pediatrician and vaccine advocate who served on advisory boards for both the CDC and the Food and Drug Administration, told CNBC earlier this month that the US is still “undervaccinated” and about half the population is not fully vaccinated be .

Even people who are fully protected have cause for concern when it comes to variants of Covid, Offit said. While the vaccines protect well against serious illness and death, they may not protect as well against minor illness or the spread of Covid to others, he said. No vaccine is 100% effective, he noted.

“It is not a bold prediction to believe that SARS-CoV-2 will be circulating in two or three years. I mean, there are 195 countries out there, most of which haven’t received a single dose of vaccine. ”“ Offit said. “Will it still be circulating in the United States? I think that would be very, very likely.”

Israel released preliminary data last week showing that the Pfizer vaccine was only 39% effective against the virus there, which officials attributed to the rapidly spreading Delta variant. Its effectiveness against serious illness and death remained high, the data showed. US and World Health officials said they would look at Israeli research, which was non-peer-reviewed and had few details.

Pfizer, Moderna, and Johnson & Johnson executives have stated that they expect Americans to need booster vaccinations, and Pfizer has announced it will ask the FDA to approve booster vaccinations as it sees signs of waning immunity. Federal health officials say that otherwise healthy people don’t currently require booster doses of the vaccines, although they may recommend it for the elderly or those with compromised immunity.

– CNBC’s Meg Tirrell and The Associated Press contributed to this report.