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(AP/WJW) — The nation’s top health agency has officially backpedaled on its masking guidelines and recommends that even vaccinated people wear masks indoors in parts of the U.S. where the coronavirus is surging.

Earlier today, a federal official had spoken on a condition of anonymity that the reversal would take place. Tuesday afternoon, the Centers for Disease Control and Prevention announced the change in a media session.

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 It is unpublished, and the CDC has not released it.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.

Dr. Anthony Fauci previously said the CDC was weighing revising their COVID-19 guidelines.

A high transmission or high surge area is a location where there are 50 cases of COVID per 100,000 people through a 7-day period.

Find the CDC’s COVID-19 data tracker map, which locates case rate by county, right here.