Roughly three-fourths of the people who became infected with COVID-19 during a recent outbreak in Massachusetts were already fully vaccinated against the virus, according to data from the Centers for Disease Control and Prevention released Friday.

The CDC’s Morbidity and Mortality Weekly Report shows that 74% of the 469 infected in the outbreak, which were linked to the popular tourist location Provincetown in Cape Cod, were fully vaccinated before they became ill, and of those who were vaccinated and contracted COVID-19, 274 displayed symptoms with four requiring hospitalization.

The delta variant was identified in 90% of specimens taken from 133 patients. The report also states that this data is ''insufficient'' for any conclusions to be made about the COVID-19 vaccines or their effectiveness against the delta variant.

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

Dr. Sabrina A. Assoumou, assistant professor of medicine at the Boston University School of Medicine, told The Boston Globe on Friday that ''With other variants, we had evidence that vaccination could prevent transmission. Therefore, it is possible that findings in the Provincetown cluster might be explained by the presence of a more contagious variant that can be transmitted by vaccinated individuals who are spending a considerable time indoors.''

State Sen. Julian Cyr, whose district includes the outer region of the Cape, said that ''there’s a real culture of healthcare-seeking behavior,'' due to Provincetown’s status as an LGBTQ landmark and the community’s experience with the AIDs epidemic.

The senator noted that other outbreaks are happening in other parts of the state and the country, but the data on those incidents is less reliable.

''What we’re learning is that if you’re vaccinated, you can become sick,'' Cyr said, adding that ''were this cluster to have happened pre-vaccination or among a population with low vaccination levels, you would see a significant number of hospitalizations and deaths.''

Dr. Robert Horsburgh, a Boston University professor of epidemiology, told the Globe that ''this is what we expect in a highly vaccinated population. Even though the COVID vaccines are very effective, they are not perfect. Therefore, whenever a disease is spreading in a community with a highly vaccinated population, the majority of cases are among vaccinated people (because there are not that many unvaccinated people).''

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