CDC changes guidelines for masking and testing due to more evidence confirming the Delta variant Covid infection in the vaccinated can occur at a high levels and vaccinated individuals have higher viral titers and are contagious.

https://www.npr.org/sections/coronavirus-live...k-guidance

"The study details a COVID-19 outbreak that started July 3 in Provincetown, Mass., involving 469 cases. It found that three-quarters of cases occurred in fully vaccinated people. Massachusetts has a high rate of vaccination: about 69% among eligible adults in the state at the time of the study.

It also found no significant difference in the viral load present in the breakthrough infections occurring in fully vaccinated people and the other cases, suggesting the viral load of vaccinated and unvaccinated persons infected with the coronavirus is similar.

The CDC said the finding that fully vaccinated people could spread the virus was behind its move to change its mask guidance.

"High viral loads suggest an increased risk of transmission and raised concern that, unlike with other variants, vaccinated people infected with Delta can transmit the virus," Dr. Rochelle Walensky, the CDC's director, said in a statement Friday."

https://www.npr.org/sections/coronavirus-live...k-guidance