Centers for Disease Control and Prevention (CDC) is set on Tuesday to recommend fully vaccinated Americans wear masks indoors in at least some instances, as the more highly transmissible Delta variant of the coronavirus has led to a surge in infections, sources said.

The CDC will announce revised mask guidance but the specifics of when the agency will recommend fully vaccinated Americans wear masks indoors are not fully clear, the sources said.

In May, the CDC advised that fully vaccinated people do not need to wear masks outdoors and can avoid wearing them indoors in most places, guidance the agency said would allow life to begin to return to normal.

CDC Director Rochelle Walensky said in May that the new guidance was based on a sharp reduction in cases, expansion of vaccines to younger people, and vaccine efficacy against coronavirus variants.

At the time, it still recommended vaccinated people wear masks on planes and trains, and at airports, transit hubs, mass transit and places like hospitals and doctor’s offices.

infectious disease official Anthony Fauci said on Sunday on CNN that health officials were considering whether to revise mask guidance for vaccinated Americans, saying it was “under active consideration.”.