By David Shepardson and Julie Steenhuysen

WASHINGTON (Reuters) -Americans fully vaccinated against COVID-19 should go back to wearing masks in indoor public places in regions where the coronavirus and especially the Delta variant are spreading rapidly, U.S. authorities said on Tuesday.

The U.S. Centers for Disease Control and Prevention (CDC) also recommended all students and teachers at kindergarten through 12th grade schools wear masks regardless of vaccination status. The CDC said children should return to full-time, in-person learning in the fall with proper prevention strategies.

The changes mark a reversal of the CDC’s announcement in May that prompted millions of vaccinated Americans to shed their face coverings.

The United States leads the world in the daily average number of new infections, accounting for one in every nine cases reported worldwide each day. The seven-day average for new cases has been rising sharply and stands at 57,126, still about a quarter of the pandemic peak.

“In areas with substantial and high transmission, CDC recommends that fully vaccinated individuals wear a mask in public indoor settings to help prevent spread of Delta and protect others,” the agency said.

The CDC said that 63% of U.S. counties had high transmission rates that warranted mask wearing. The Delta variant is highly transmissible.

In May, the agency advised that fully vaccinated people do not need to wear masks outdoors and can avoid wearing them indoors in most places, guidance the agency said would allow life to begin to return to normal.

Dr. David Doudy, an epidemiologist at Johns Hopkins University, said the CDC guidance was motivated by a change in infection patterns. “We’re seeing this doubling in the number of cases every 10 days or so,” he said.

‘A NECESSARY PRECAUTION’

American Federation of Teachers President Randi Weingarten praised the new CDC mask guidance in a statement, saying it was needed “to deal with the changing realities of virus transmission.”

She called it “a necessary precaution until children under 12 can receive a COVID vaccine and more Americans over 12 get vaccinated.”

The new CDC recommendations are not binding and many Americans, especially in Republican-leaning states, may choose not to follow them. At least eight states bar schools from requiring masks.

Dr. Isaac Weisfuse, a medical epidemiologist and adjunct professor at Cornell University Public Health, said resistance was likely among some people. “I think we will get blowback because I think people might view it as backtracking,” he said.

On Monday, the Biden administration confirmed it will not lift any existing international travel restrictions, citing the rising number of COVID-19 cases and the expectation that they will continue to rise in the weeks ahead.

Masks became a political issue in the United States under former President Donald Trump, who resisted mandating face coverings. President Joe Biden has embraced masks and mandated them for transit hubs days after taking office.

(Reporting by David Shepardson; Additional reporting by Nandita Bose; Editing by Rosalba O’Brien, Bill Berkrot and Cynthia Osterman)