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The Centers for Disease Control and Prevention is expected to revise 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 is expected to come at a 3 p.m. briefing. It isn’t yet clear how substantial or high transmission will be determined. 

The change comes as the Delta variant sweeps across the country, although health officials still insist the approved vaccines give strong protection against the most serious effects of the virus. Up to this point, federal health officials had not made recommended changes for vaccinated people since announcing in May that fully vaccinated people do not need to wear masks indoors.

Masks are still required even for fully vaccinated travelers on buses, planes and other forms of public transportation. 

The Department of Veterans Affairs on Monday became the first federal agency to require employees to be vaccinated, with the VA secretary announcing all medical personnel will be required to be fully vaccinated in the next eight weeks. 

This is a developing story. Please check back for updates.