The Centers for Disease Control and Prevention (CDC) on Tuesday essentially reversed earlier COVID-19 guidance by saying fully vaccinated people should now wear masks in certain situations.

The agency said fully vaccinated people should wear masks in “public, indoor settings” in areas of the country with “substantial” or “high” levels of transmission, as defined by the CDC. Those areas currently include much of the South and West.

The CDC also said all adults and students should wear masks in K-12 schools regardless of vaccination status.

The new guidance contrasts with coronavirus recommendations from May, when the CDC said fully vaccinated people did not need to wear masks, except in a few circumstances.

CDC Director Rochelle WalenskyRochelle WalenskyCNN: Every county in Florida, Arkansas rated ‘high transmission’ for community spreadHouse Democrats expand probe into political interference into CDC during Trump administrationNo one wants to say it: We need to mask up againMORE on Tuesday said the change was needed because of new science showing that with the delta variant, some vaccinated people could transmit the virus to others on “rare occasions.”

“This new science is worrisome and unfortunately warrants an update to our recommendations,” Walensky said.

The CDC is still urging unvaccinated people to get the shot, saying it continues to overwhelmingly protect against severe outcomes of the disease, even with the delta variant.