ATLANTA — The Centers for Disease Control and Prevention recommended on Tuesday that Americans fully vaccinated against the coronavirus should start wearing masks indoors again in parts of the country where transmission of the more infectious Delta variant is particularly high.

This follows a 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.

The new guidance announced by CDC director Rochelle Walensky during an afternoon briefing, comes after top officials gathered Sunday night to review new data and evidence regarding the transmissibility of the Delta COVID-19 variant and a growing number of breakthrough cases.

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, Dr. 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, she added.

The updated guidance reverses a recommendation from the agency in May that said fully vaccinated people would no longer need to wear masks in most indoor settings.