Among 469 COVID-19 infections tied to a number of summer events and large public gatherings in Barnstable County, Mass., about three-quarters of cases occurred in fully vaccinated people, according to the CDC's latest Morbidity and Mortality Weekly Report published July 30.
A number of large public gatherings occurred at a town in the county between July 3-17 and beginning July 10, the state's health department started receiving reports of an uptick in COVID-19 infections among fully vaccinated residents. Of 469 infections associated with the events, about 74 percent were breakthrough infections, according to the report.
A total of five people were hospitalized, including four who were fully vaccinated. No deaths were reported.
The outbreak was largely fueled by the delta variant, first detected in India and now the dominant U.S. strain. The variant was detected in 89 percent of specimens that were sequenced from 133 patients.
Additionally, specimens from vaccinated people who were infected had similar cycle threshold values to those who were unvaccinated and infected. "This might mean that the viral load of vaccinated and unvaccinated persons infected with SARS-CoV-2 is also similar," researchers said.
"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," the report said.
This report was part of previously unpublished data that contributed to the CDC's recent decision to update mask guidance, which suggests everyone wear a mask in public indoor spaces in areas of high or substantial community transmission, regardless of vaccination status.
"This finding is concerning and was a pivotal discovery leading to CDC's updated mask recommendation," Rochelle Walensky, MD, CDC director, said in a July 30 statement. "The masking recommendation was updated to ensure the vaccinated public would not knowingly transmit virus to others, including their unvaccinated or immunocompromised loved ones."