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They came from across Massachusetts and around the country to celebrate the start of summer and the loosening of pandemic restrictions. But, before long, the revelers learned they had unwittingly triggered the first known major outbreak of COVID-19 among a highly vaccinated group of people.

Now a new study confirms what researchers had suspected, that the gathering in Provincetown, despite its ideal circumstances for triggering a massive number of infections, did not become a super-spreader event around the country. Moreover, the study is the first to trace how the outbreak started in Provincetown, where more than 1,000 people in Massachusetts were infected.

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The study found that the Delta variant that spread through Provincetown came from more than 40 different sources outside the Cape Cod community. And just one of those, from either an individual or small group, seeded 83 percent of the infections studied.

The outbreak could have erupted into a super-spreader event similar to the one set off at Boston’s February 2020 Biogen conference. But instead it subsided relatively quickly with only a handful of people getting sick enough to be hospitalized.

“There is no reason this couldn’t have ended up the size of the Biogen event,″ said Samuel Scarpino, managing director of pathogen surveillance at the Rockefeller Foundation’s Pandemic Prevention Institute.

“The reason it didn’t is that the community and public health officials responded decisively and leveraged the tools we know work: masking, quarantine, and testing to bring this under control,” he said.

The team of researchers, led by scientists at the Broad Institute of MIT and Harvard, concluded that Cape Cod’s high vaccination rate and quick public health measures in Massachusetts likely prevented the outbreak from erupting into many more infections.

“Cases were climbing quickly [in Massachusetts] and we might have thought the outbreak in Provincetown accelerated or caused that, but this genomic data, in black and white, show us that is not true,” said Bronwyn MacInnis, director of pathogen genomic surveillance at the Broad and a co-lead author of the study, which was published Wednesday on the preprint server medRxiv but has not yet been peer reviewed.

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MacInnis is part of the same team that published a landmark study on the Provincetown outbreak in late July that prompted the US Centers for Disease Control and Prevention to reverse course and recommend vaccinated people resume wearing masks in crowded places. That study found that vaccinated people who become infected can carry as much of the virus as those who are unvaccinated.

Like detectives, the researchers immediately started their genomic sleuthing of cases as infections exploded in Massachusetts in the weeks after the July Fourth celebrations on the Cape. Their current study expands on their July work; they combined contact tracing, interviewing those infected and their close contacts, with genome sequencing, scrutinizing thousands of molecules in the respiratory samples taken from 467 of those infected, to track how the outbreak started and then mushroomed so quickly.

Masks were required indoors in Provincetown in August.
Masks were required indoors in Provincetown in August.Craig F. Walker/Globe Staff

They traced the origin of the outbreak to about 40 sources, but could not say whether these were individuals or small clusters of people.

But just one of these 40 sources, the researchers found, was responsible for 83 percent of the infections studied. This large cluster was likely not the result of extensive spread at a single site, such as one of the packed nightclubs that week, but rather a series of transmissions from a common source across multiple settings.

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The researchers at that time were worried the rapid spread in the Provincetown outbreak signaled a possible new and turbocharged version of the Delta variant that could be even more transmissible. But the team found that 99 percent of the cases they sequenced were the same Delta variant that had been circulating elsewhere.

“Genomics can add another perspective and extra resolution to show how effective the Massachusetts public health interventions were,” said co-lead author Katherine Siddle, who also is a researcher at the Broad.

Provincetown health officials had reimposed an indoor mask mandate days before the team’s July report was released, and also greatly increased COVID testing to catch additional infections before they could spread.

The Boston Marriott Long Wharf hosted the Biogen leadership conference in February 2020.
The Boston Marriott Long Wharf hosted the Biogen leadership conference in February 2020.Erin Clark/Globe Staff

The Biogen conference emerged as one of the main drivers of the COVID-19 pandemic, linked to as many as 300,000 COVID infections worldwide, as executives who were exposed to the disease in Boston returned to their far-flung homes.

A critical difference between the Biogen and Provincetown outbreaks was vaccinations. The vast majority of the Provincetown revelers with reported infections had received their shots.

But the Biogen event pre-dated COVID vaccines, and it unfolded before public health officials realized the virus was transmitted through aerosols and could be blunted by wearing masks and staying physically distant.

The Broad team also tracked the impact of the Provincetown outbreak more widely in the United States by searching for traces of it in national surveillance data collected between July 10 and Sept. 13. The team found signs of transmission from between 18 and 37 states, including New York, California, and Georgia, but found that these cases represented less than 1 percent of the more than 400,000 Delta genomes collected nationally in the same period.

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“These findings suggest that, while the outbreak led to some onward transmission, it made at most a modest contribution to later Delta cases within MA and a minimal contribution to cases elsewhere in the US,” the study said.

Commerce Street in Provincetown in August.
Commerce Street in Provincetown in August.Craig F. Walker/Globe Staff

Shan Soe-Lin, a lecturer at Yale University’s Jackson Institute for Global Affairs, said she was comforted by the findings of the study, which she described as, “very careful, very thorough, and the genomics were super cool.”

She said the study underscored that vaccines work to protect legions of people from serious illness.

“These were thousands of people crowded in really small spaces dancing and partying and all kinds of things that had extremely close contact for hours,” said Soe-Lin, also a managing director at Pharos Global Health Advisors, a Boston global health advisory firm. “This was really pushing vaccines to the limit.”

She said the study’s findings highlight the need for the country to be regularly employing the same type of genomic detective work to spot and stay ahead of concerning variants in COVID and other viruses to keep communities safe.

“The US is still flying blind on sequencing for this virus, or any virus,” she said.

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Kay Lazar can be reached at kay.lazar@globe.com Follow her on Twitter @GlobeKayLazar.