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How much worse would Provincetown’s COVID-19 outbreak have been without vaccines?

Provincetown may have escaped becoming a ‘Biogen 2.0′

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

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At first blush, a wintertime conference in downtown Boston and a series of summer parties at the tip of Cape Cod don’t seem all that similar.

But major COVID-19 outbreaks in the two settings nearly a year and a half apart dramatically demonstrate the protective value of vaccinations — even in the face of a more contagious form of the virus. The February 2020 Boston Biogen conference set off a global tidal wave of COVID infections, while the outbreak at the Provincetown parties appeared to fizzle with only a handful getting sick enough to be hospitalized.

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The big difference? Vaccinations. The vast majority of the Provincetown revelers with reported infections had received their shots.

If not for all the vaccinations, warned Dr. Ashish Jha, dean of Brown University’s School of Public Health, “that would have been a disaster.”

The infamous Biogen conference of 175 executives from around the world shares several striking similarities with the recent July Fourth celebrations in Provincetown, which attracted visitors from across the country. The people at both gatherings were relatively young, mobile — and vulnerable.

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

During the conference, in the earliest days of COVID-19, scientists hadn’t yet realized masks and social distancing could curb the virus’s spread. In Provincetown, many also gathered closely and without masks because they were vaccinated and following CDC guidance at that time, which suggested such precautions weren’t needed.

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.

But it appears Provincetown has not become a Biogen 2.0, at least from what is now known, even though it had the makings of an even bigger superspreader event, one involving thousands of people packing house parties and clubs during weeklong festivities.

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By the end of July, roughly 1,000 COVID-19 infections were linked to the events in Provincetown. Three-quarters of the patients analyzed by the Centers for Disease Control and Prevention were vaccinated. In all, eight people were hospitalized, but no one died.

Disease experts point to Provincetown as a remarkable stress test, saying that high vaccination rates in the town and surrounding Barnstable County substantially dampened what could have been much worse.

Jha’s admittedly very rough estimate is that, without any vaccines, the initial caseloads would have been at least five times larger than they were.

Jha based his calculation on the CDC’s estimate that the Pfizer and Moderna shots prevent about 80 percent of symptomatic infections from the wildly contagious Delta variant. So instead of 20 percent infected with symptoms, even with vaccines, it would have been about five times that without vaccines, under Jha’s estimate. That would have meant 5,000 initial infections, instead of the roughly 1,000.

“The July Fourth weekend in Provincetown, if that population was not vaccinated, would have led to many tens of thousands of infection over time,” Jha said.

“It would have been a Biogen,” he said.

Jacob Lemieux, an infectious disease physician at Massachusetts General Hospital and lead researcher on the Biogen study, has now joined with colleagues at the Broad Institute of MIT and Harvard, and the CDC to sequence infections from the Provincetown outbreak and, once again, follow the path of transmission.

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Lemieux said scientists are still trying to understand the impact from the Delta strain, which is at least twice as contagious as the original COVID-19 virus. The Provincetown outbreak demonstrated that, while vaccinated people were unlikely to get seriously ill from Delta, they can carry it, potentially threatening others who are more vulnerable.

“Provincetown is a wake-up call,” he said.