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

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

The White House quickly pivoted on its own masking guidance, asking all staff and reporters to wear masks indoors because the latest CDC data shows that Washington faces a substantial level of coronavirus transmission.

But with the delta variant, a mutated and more transmissible version of the virus, 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.

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

For much of the pandemic, the CDC advised Americans to wear masks indoors and 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 guidance was eased further, allowing fully vaccinated people to stop wearing masks outdoors in crowds and in most indoor settings.

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

Unvaccinated people who did not want to wear masks in the first place saw it as an opportunity to do what they wanted, they said.

"If all the unvaccinated people were responsible and wore mask indoors, we would not be seeing this surge," said Dr.

Furthermore, it's not likely to change the behavior of the people who most need to wear masks.

Ken Thigpen, a retired respiratory therapist who now works for a medical device manufacturer, is fully vaccinated and stopped wearing his mask in public in May.