WASHINGTON (CBS News) — The U.S. Centers for Disease Control and Prevention revised its mask guidance Tuesday afternoon, saying fully vaccinated Americans should wear masks indoors in communities where there is substantial or high transmission, according to a federal official.

The announcement was officially made at a 3 p.m. ET briefing. The new guidance comes as a growing number of local and state health officials have already returned to mandating masks indoors, with cases of the virus surging among unvaccinated Americans. Some 63% of U.S. counties currently have a “high” or “substantial” spread of the virus, according to the CDC.

CDC Director Rochelle Walensky also said K-12 schools should require masks indoors, regardless of students’ and teachers’ vaccination status.