The Centers for Disease Control and Prevention has a lot of ways to pick up on COVID-19 outbreaks, but those methods often take awhile to bear fruit. Not so with the Provincetown, Mass., cluster that started around July Fourth weekend. “We triggered the investigation as people were getting symptomatic,” says Demetre Daskalakis, a deputy incident manager for the CDC’s COVID-19 Response. “Pretty amazing — it is warp speed.” How did they do that? It was thanks to a tip from a citizen scientist named Michael Donnelly. A data scientist in New York City’s tech sector, he started publishing his own coronavirus data reports early in the pandemic and launched a website, COVIDoutlook.info, with Drexel University epidemiologist Michael LeVasseur. Following leads from his personal network, Donnelly documented over 50 breakthrough cases coming out of Provincetown, practically in real time, and shared it with the CDC as the outbreak was still unfolding. Without Donnelly’s effort, the agency would have probably detected the outbreak at some point, Daskalakis says, but “it wouldn’t have been as rapturous an initiation of an investigation and response as we had.” The speed of the investigation — and the exceptional participation from the mostly gay men involved in the outbreak — helped the CDC learn new information about the delta variant. And it was that new information, in part, that prompted the agency to change its guidance for how vaccinated people should keep themselves safe at this stage of the pandemic — including a return to masking indoors. It’s a testament to the power of citizens engaging with the scientific process, Daskalakis says. “I get goose bumps thinking about it,” he says. “Community plus public health is magic.” Donnelly didn’t go to Provincetown with his husband for July Fourth, but his friends who were there told him all about it. As they do every year, thousands of gay men arrived for the holiday in this little artist community on the tip of Cape Cod to rent cottages, go to the beach and drag shows and restaurants, and crowd into nightclubs to dance. As the festivities from the July Fourth week wrapped up, the updates and gossip streaming into Donnelly’s phone quickly took a different cast from years’ past. On July 9, he texted some close friends, “[D]id you boys survive the Fourth in P-town?” The response: “Our entire house can’t stop coughing.” More texts started coming in, including from fully vaccinated friends testing positive for the coronavirus. He started thinking to himself, he says, that given that vaccination rates were really high among the gay community and in Provincetown, “the odds don’t really add up.” With the pandemic keeping friends apart for so long, there was extra excitement this year, says Zorik Pesochinsky, who traveled to Provincetown from New York City. “The lines were even longer — the bars were even busier and more full,” he says. He and everyone else he knew going were fully vaccinated, so he felt safe. “I was definitely going into it with a mindset of, this is all behind us, we’re just going into a super-fun, amazing weekend.” It was a rainy week, which meant everyone was indoors more. “It would get so incredibly hot in these clubs that you would just be wet with sweat, so you’d have to step outside for a moment just to get a breath of fresh air,” says Sean Holihan, who came from Washington, D.C. Halfway through the week, when a few of Cameron Thomas’ vacation housemates started coughing, he didn’t think much of it. “You’re saying hi to so many people, you’re in situations where you don’t sleep a lot, you’re running around — you’re going to catch something,” he says. But it wasn’t a summer cold that was going around. By the end of that week, news of breakthrough COVID-19 cases started to roll in. And they kept coming.