Nearly 75% of COVID-19 infections during a July outbreak in a Massachusetts town happened to people who were previously vaccinated, the CDC said in a study released Friday.

“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,” the CDC report said. “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.” (RELATED: CDC Finally Shows The Public The Data Behind Its New Mask Guidance)

Graphics: A closer look at the Cape Cod COVID-19 cluster data https://t.co/mjvk62YP9c

— The Boston Globe (@BostonGlobe) July 31, 2021

The study was based on 469 cases of COVID-19 associated with several outdoor events in July in Provincetown, Massachusetts, according to NBC News. The patients studied are those who completed a two-dose course of the mRNA vaccines or received a single shot of Johnson & Johnson’s. The CDC reported that 274 vaccinated people with an infection had symptoms such as a cough, headache, sore throat, muscle pain and fever. Five of the patients were hospitalized, four of whom were fully vaccinated, according to the agency. No deaths were reported. Testing revealed the delta variant was in 90% of specimens from 133 patients.

Delta has evolved into the dominant form of the disease in the United States, according to the CDC.

As a result of the latest surge in cases, the Biden administration and the CDC have reversed previous instructions about wearing masks for those that have been vaccinated.

“On July 27, CDC recommended that all persons, including those who are fully vaccinated, should wear masks in indoor public settings in areas where COVID-19 transmission is high or substantial,” the CDC published in their study. “Findings from this investigation suggest that even jurisdictions without substantial or high COVID-19 transmission might consider expanding prevention strategies, including masking in indoor public settings regardless of vaccination status, given the potential risk of infection during attendance at large public gatherings that include travelers from many areas with differing levels of transmission.”

“This finding is concerning and was a pivotal discovery leading to CDC’s updated mask recommendation,” CDC Director Dr. Rochelle Walensky said, according to CNBC. “The masking recommendation was updated to ensure the vaccinated public would not unknowingly transmit virus to others, including their unvaccinated or immunocompromised loved ones.”