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The U.S. Centers for Disease Control and Prevention (CDC) now recommends vaccinated people in certain parts of the country wear masks while indoors, reversing a decision the agency made two months ago.
In another reversal, the CDC also now recommends teachers, students and staff wear masks inside schools, regardless of vaccination status.
The decision to recommend masks indoors, the CDC says, was based on surging COVID-19 case numbers in parts of the country reporting low vaccination rates.
The rising caseload is driven by the highly contagious delta variant of the coronavirus that causes COVID-19. There have also been a number of "breakthrough" cases in which fully vaccinated people are infected, though they generally become less sick.
According to CDC Director Dr. Rochelle Walensky, with earlier variants of the virus, vaccinated people showed low levels of the virus and were considered unlikely to be spreaders.
But with the delta variant, she said, the levels of the virus found in the noses and throats of infected vaccinated people was "indistinguishable" from infected unvaccinated people.
"Today's announcement by the CDC ... is another step on our journey to defeating this virus," President Joe Biden said in a White House statement. "I hope all Americans who live in the areas covered by the CDC guidance will follow it; I certainly will when I travel to these areas."
Health officials say vaccines remain effective against the worst outcomes of infection with the virus, including those involving the delta variant.
In May, the CDC said fully vaccinated people no longer would be required to wear masks or maintain social distancing of six feet from other people. The agency still suggested people remain masked on public transportation and at crowded outdoor events.
For months, COVID-19 cases, deaths and hospitalizations in the U.S. fell steadily, but those trends reversed over the past two months as the delta variant of the coronavirus began to spread.
The New York Times reports several cities and towns have restored indoor masking rules in recent weeks, including St. Louis, Missouri; Savannah, Georgia; and Provincetown, Massachusetts.
Some information for this report was provided by the Associated Press, Reuters and the French News agency, AFP.