An investigation by the Massachusetts Department of Public Health, along with the Centers for Disease Control and Prevention, found that 882 people were connected to the recent Provincetown outbreak.
Of those individuals who were living in Massachusetts, 74 percent were vaccinated but reported experiencing symptoms, according to ABC News.
Vaccinated people who do get COVID-19 are "breakthrough infections."
The Cape Cod beach town saw a rise in COVID cases after the July 4 holiday weekend. The summer is high season for Provincetown, a favorite destination for LGBTQ+ people, with hundreds packing bars, restaurants, and outdoor events like tea dances.
Health officials had thought that it was a rarity for vaccinated people to get COVID-19. And even if they were infected with the novel coronavirus, then they couldn't really spread it to others. It was that assumption, ABC News reported, that made the CDC loosen its masking recommendations in May.
However, the data the CDC used to make those judgments were from earlier studies that did not include the Delta variant. Health officials said that the virus has changed.
Provincetown's numbers now offer evidence for the CDC's recent decision to recommend masking indoors.
"In recent days I have seen new scientific data from recent outbreak investigations, showing that the Delta variant behaves uniquely differently from past strains of the virus that causes COVID 19," CDC Director Rochelle Walensky told reporters Tuesday.
"Information on the Delta variants from several states and other countries indicate that on rare occasion some vaccinated people infected with a Delta variant after vaccination may be contagious and spread the virus to others," Walensky said. "This new science is worrisome and unfortunately warrants an update to our recommendation."
On Thursday night, The Washington Post reported on internal CDC documents that indicate "the war has changed" when it comes to the pandemic.
"The document strikes an urgent note, revealing the agency knows it must revamp its public messaging to emphasize vaccination as the best defense against a variant so contagious that it acts almost like a different novel virus, leaping from target to target more swiftly than Ebola or the common cold," the Post reported.