Large public gatherings are associated with SARS-CoV-2 breakthrough infections—detectable virus in fully vaccinated people—according to a report the US Centers for Disease Control and Prevention released Friday (July 30). According to multiple outlets, the data prompted the agency’s shift earlier last week to again recommending that vaccinated people wear a mask indoors in some circumstances.

The report focuses on 469 COVID-19 cases identified in Barnstable County, Massachusetts during July 2021 that were sparked by public summer gatherings. Approximately three-quarters of the cases were breakthrough infections, and of the cases that were DNA-sequenced, 90 percent were caused by the Delta variant.

In addition to the startling number of cases in fully vaccinated people, the report found that the vaccinated patients had comparable viral loads to the unvaccinated patients, which could indicate a capacity for transmission. Although this finding “just gives you an indication of how much viral RNA is in the sample, it tells you nothing about infectiousness,” University of Southern California clinical microbiologist Susan Butler-Wu tells Science NewsShe says that if an infected vaccinated person develops symptoms, “we already knew that they’d have higher viral loads [than asymptomatic cases],” but that high viral loads are not necessarily indicative of infectiousness. Yale University virologist Brett Lindenbach adds that the virus particles may be coated in antibodies, and therefore could be less infectious, but officials don’t yet have the data to be sure.

Nevertheless, CDC Director Rochelle Walensky cited the high viral loads as the impetus for the CDC’s updated mask recommendations in a statement last week. “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,” Walensky said Friday, according to the Journal“This finding is concerning and was a pivotal discovery leading to CDC’s updated mask recommendation,” which urges everyone—vaccinated and unvaccinated—to wear masks in indoor public spaces in regions with high rates of Delta infection and in K-12 schools regardless of local infection rates.

Additionally, internal documents from the CDC originally published by the The Washington Post say the Delta variant is more transmissible than the 1918 pandemic flu and as transmissible as chicken poxNew studies this summer from Singapore, Scotland, and Canada suggest that in addition to being more transmissible, the Delta variant is associated with an increased risk of severe COVID-19, including hospitalization and death. As of today (August 2), the studies out of Canada and Singapore were preprints and had not yet been peer-reviewed.

While the variant appears to be driving an increase in breakthrough infections, The New York Times notes that 97 percent of people hospitalized for COVID-19 across the United States are unvaccinated, underscoring the vaccines’ efficacy against severe illness. In the Massachusetts data, 274 of the vaccinated patients experienced fairly mild symptoms such as fever, sore throat, cough, and headache, says the Times; only four of them were hospitalized, and there were no deaths.