The Centers for Disease Control and Prevention announced new recommendations that vaccinated people return to wearing masks indoors in parts of the U.S. where the coronavirus is surging.

Scientists cited 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 high in the South.

The country is averaging more than 57,000 cases a day and 24,000 COVID-19 hospitalizations. Most new infections and hospitalizations in the U.S. are 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 more transmissible delta variant, “it is concerning enough that we feel like we have to act,” Walensky said regarding a return to wearing masks.