https://www.npr.org/sections/health-shots/2021/07/28/ [login to see] /cdc-mask-guidelines-indoors-vaccinated-by-county-covid-spread
The change to the Centers for Disease Control and Prevention's masking guidelines came after pressure from many outside experts. The CDC's director, Dr. Rochelle Walensky, said in a news briefing that the change comes after new evidence showed delta was more transmissible than previously understood.
"This was not a decision that was taken lightly," Walensky said. She noted that new data from outbreak investigations show that, rarely, vaccinated people can still get infected and spread the virus to others.
"On rare occasions, some vaccinated people infected with the delta variant after vaccination may be contagious and spread the virus to others," she told reporters when announcing the new guidelines. "This new science is worrisome and, unfortunately, warrants an update to our recommendations."