The Centers for Disease Control and Prevention announced in May that fully vaccinated individuals didn’t need to wear masks in most settings — but that guidance changed Tuesday in light of new data and rising cases.

Now even fully vaccinated Americans should wear masks indoors in areas of high COVID-19 transmission, according to the new guidance. 

Although the majority of new infections in the U.S. are in unvaccinated people, “breakthrough” infections, which generally cause milder illness, can occur in vaccinated people. The new guidance was in part prompted by data showing that the highly-transmissible delta variant could possibly be spread by vaccinated people, according to the CDC.

Many state and local government are in the process of responding to the new announcement but, in many, mask guidelines still aren’t aligned with transmission rates. Because the U.S. generally considers public health a state matter, states are not obligated to immediately implement the guidance.

Since May, most states have backed off mask requirements for vaccinated people and some lifted their mask mandates for everyone. A few never had any requirements in the first place. 

Ahead of the CDC’s announcement, several local governments — including Los Angeles County and St. Louis — began requiring vaccinated people to again wear masks, citing concerns about the delta variant.