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The Centers for Disease Control and Prevention reversed course Tuesday on some masking guidelines, recommending that even vaccinated people return to wearing masks indoors in parts of the U.S. where the coronavirus is surging.

Citing new information about the ability of the delta variant to spread among vaccinated people, the CDC also recommended indoor masks for all teachers, staff, students and visitors to schools, regardless of vaccination status.

The new guidance follows recent decisions in Los Angeles and St. Louis to revert to indoor mask mandates amid a spike in COVID-19 cases and hospitalizations that have been especially bad in the South. The country is averaging more than 57,000 cases a day and 24,000 COVID-19 hospitalizations.

Most new infections in the U.S. continue to be among unvaccinated people. But “breakthrough” infections, which generally cause milder illness, can occur in vaccinated people. When earlier strains of the virus predominated, infected vaccinated people were found to have low levels of virus and were deemed unlikely to spread the virus much, CDC Director Dr. Rochelle Walensky said.

But with the delta variant, the level of virus in infected vaccinated people is “indistinguishable” from the level of virus in the noses and throats of unvaccinated people, Walensky said.

The data emerged over the last couple of days from 100 samples. It is unpublished, and the CDC has not released it. But “it is concerning enough that we feel like we have to act,” Walensky said.

Vaccinated people “have the potential to spread that virus to others,” she said.

“The Oklahoma State Department of Health has and will continue to follow CDC guidance regarding COVID-19. With today’s revisions by the CDC, we want to reiterate the importance of taking personal responsibility for protection against COVID-19. Additionally, we continue to encourage those who are eligible to get vaccinated.” 

State Epidemiologist Jolianne Stone

For much of the pandemic, the CDC advised Americans to wear masks outdoors if they were within 6 feet of one another.

Then in April, as vaccination rates rose sharply, the agency eased its guidelines on the wearing of masks outdoors, saying that fully vaccinated Americans no longer needed to cover their faces unless they were in a big crowd of strangers. In May, the CDC further eased its guidance for fully vaccinated people, allowing them to stop wearing masks outdoors in crowds and in most indoor settings.

The guidance still called for wearing masks in crowded indoor settings, like buses, planes, hospitals, prisons and homeless shelters, but it cleared the way for reopening workplaces and other venues.

Subsequent CDC guidance said fully vaccinated people no longer needed to wear masks at summer camps or at schools, either.

For months COVID cases, deaths and hospitalizations were falling steadily, but those trends began to change at the beginning of the summer as a mutated and more transmissible version of the coronavirus, the delta variant, began to spread widely, especially in areas with lower vaccination rates.

In recent weeks, a growing number of cities and towns have restored indoor masking rules. St. Louis, Savannah, Georgia, and Provincetown, Massachusetts, are among the places that reimposed mask mandates this month.