The Centers for Disease Control and Prevention recommended Tuesday that fully vaccinated people begin wearing masks indoors again in places with high Covid-19 transmission rates. The agency is also recommending kids wear masks in schools this fall, according to CNBC.

Federal health officials still believe fully vaccinated people represent a very small amount of transmission. Still, some vaccinated people could be carrying higher levels of the virus than previously understood and potentially transmit it to others.

“This pandemic continues to pose a serious threat to the health of all Americans,” CDC Director Rochelle Walensky told reporters on a call. “Today, we have new science related to the delta variant that requires us to update the guidance regarding what you can do when you are fully vaccinated.”

The updated guidance comes ahead of the fall season, when the highly contagious delta variant is expected to cause another surge in new coronavirus cases and many large employers plan to bring workers back to the office. In mid-May, the CDC said fully vaccinated people didn’t need to wear masks in most settings, whether indoors or outdoors.

“In areas with substantial and high transmission, CDC recommends fully vaccinated people wear masks in public, indoor settings to help prevent the spread of the delta variant, and protect others. This includes schools,” Walensky said. The CDC recommends that everyone in grade schools wear masks indoors, “including teachers, staff, students and visitors, regardless of vaccination status.”

Health experts fear delta, already the dominant form of the disease in the U.S., is hitting states with low vaccination rates. Those states are now being forced to reintroduce mask rules, capacity limits and other public health measures that they’ve largely rolled back in recent months.