The U.S. Centers for Disease Control and Prevention, or CDC, recommended Tuesday that fully vaccinated Americans wear face-coverings in areas "with substantial and high transmission" of COVID-19.

The CDC also advised all K-12 students, teachers, workers, and visitors to wear a face-covering, or mask. The recommendation is in effect for both unvaccinated and vaccinated people.

The health agency said the highly infectious Delta variant has led to a large increase of coronavirus cases in parts of the country. The variant now represents more than 80 percent of new cases, especially in areas where fewer people are vaccinated. The CDC's investigation also showed that, in rare cases, some vaccinated people infected with the Delta variant may "spread the virus to others."

CDC Director Rochelle Walensky added, "This new science is worrisome and, unfortunately, warrants an update to our recommendation."

The recommendation marked a sharp change of the CDC's position from two months ago. On May 13, the health agency said fully vaccinated Americans did not need to wear masks or social distance in most places.

At that time, the CDC said people should only wear face coverings in some settings like buses, airplanes and hospitals. And it said people with weak immune systems, such as those undergoing cancer treatment, should talk with their doctors before giving up their masks.

Delta variant

The Delta variant was first identified in India last March. By July, the World Health Organization said it had spread to 96 countries.

In the U.S., the CDC reported that more than 63 percent of U.S. counties are now considered to be areas "with substantial and high transmission" of COVID-19. Most of them are in the South.

On Tuesday, Walensky said, "Vaccinated individuals continue to represent a very small amount of transmission occurring around the country." She recommended that unvaccinated people get shots and asked everyone to wear masks "to help prevent the spread of the Delta variant and protect others -- this includes schools."

Speaking to reporters, U.S. President Joe Biden said, "The more we learn about this virus and the Delta variation, the more we have to be worried and concerned. And there's only one thing we know for sure — if those other 100 million people got vaccinated, we'd be in a very different world."

The Biden administration also announced that it is asking all workers and visitors to wear masks at the White House. That is because the latest CDC information shows that Washington, DC faces a substantial level of coronavirus transmission.

American Federation of Teachers President Randi Weingarten praised the agency's new guideline to wear masks in schools. She said in a statement, "It is a necessary precaution until children under 12 can receive a COVID-19 vaccination and more Americans 12 and older get vaccinated."

The new CDC guidelines, however, are only recommendations. It is up to states and local officials to follow them. And many Americans, especially in southern states, may choose not to follow them.

Dr. Isaac Weisfuse is a professor at Cornell University Public Health. He told the Reuters news agency that some people might resist. "I think we will get blowback because I think people might view it as backtracking," he said.

I'm Jonathan Evans.

Hai Do wrote this story for VOA Learning English. Mario Ritter Jr. was the editor.

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substantial –adj. large in amount or number

transmission –n. the act or process by which something is spread or passed to another person

K-12 –abbreviated adj. kindergarten through grade 12 of school

status –n. the current state of something

warrant –v. to require or deserve something

precaution –n. a measure taken to prevent possible harm

county –n. an area of a state or country that is larger than a city and has its own government to deal with local matters

blowback –n. an unexpected and unwanted effect

backtracking –n. to go back and do something that was done in the past