In July of 2021, the CDC reversed their guidance for indoor mask wearing, regardless of vaccination status.
This caused controversy, because the CDC upon changing the guidance, did not publish the data supporting it.
On July 30, 2021, they did. The below is the report which is, according to the CDC, what played a role in their viewpoint change.
According to the report:
In July 2021, following multiple large public events in a Barnstable County, Massachusetts, town, 469 COVID-19 cases were identified among Massachusetts residents who had traveled to the town during July 3–17; 346 (74%) occurred in fully vaccinated persons. Testing identified the Delta variant in 90% of specimens from 133 patients. Cycle threshold values were similar among specimens from patients who were fully vaccinated and those who were not.
Jurisdictions might consider expanded prevention strategies, including universal masking in indoor public settings, particularly for large public gatherings that include travelers from many areas with differing levels of SARS-CoV-2 transmission.
Outbreak of SARS-CoV-2 Infections, Including COVID-19 Vaccine Breakthrough Infections, Associated with Large Public Gatherings — Barnstable County, Massachusetts, July 2021 [5 Pages, 0.6MB]