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Experts on Tuesday said they suspected the highly transmissible Delta variant is fueling a large number of breakthrough cases in Provincetown that prompted officials there to issue a new mask-wearing advisory, but they cautioned the situation could have been much more dire in a community with a lower vaccination rate than the popular summer tourist destination.

The specialists also said the outbreak is proof vaccinations are working and protecting infected people from severe illness and hospitalization.

Officials in Provincetown said 132 COVID-19 cases have been reported from July 1 to July 16, and a “vast majority” of them were among vaccinated people, though their symptoms are mild.

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“The first thing I thought about it was ‘That’s Delta for you,’” said Dr. William Hanage, an associate professor of epidemiology at the Harvard T.H. Chan school of Public Health.

“Even in quite highly vaccinated communities, Delta is capable of transmitting,” Hanage said, adding that he’s concerned “we’re now at the very start of the Delta surge in Massachusetts.”

In response to the cluster, the town issued a new public health advisory on Monday that asks people to wear masks indoors, regardless of vaccination status, when they can’t social distance, and requests that “high density” businesses require proof of vaccination before allowing people to enter. Officials have not disclosed whether any of the cases were confirmed to be the Delta variant.

On Tuesday, public health officials in Boston recommended that Boston residents who have traveled to Provincetown since July 1 get tested for COVID-19, self-isolate, and avoid gatherings for at least five days, regardless of vaccination status, after 35 local cases were traced back to the cluster “and the overwhelming majority of those have been fully vaccinated.”

On Saturday, the town of Truro issued a statement saying following the July Fourth holiday, the Cape town has seen “a growing caseload” of Truro residents who have tested positive, many of whom are vaccinated. The town is working to track the travel history of the cases and see “if they relate to the Provincetown cluster.”

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Dr. Cassandra Pierre, a hospital epidemiologist at Boston Medical Center and an assistant professor at the Boston University, said she thinks the additional step of implementing an indoor mask mandate in Provincetown would be appropriate because of the possibility that unvaccinated people are transmitting the virus.

“There are concerns that I have that this event does potentially represent that unvaccinated individuals are also in the mix, removing masks, being in close proximity, being in indoor settings, and not potentially being as careful about risk and exposure,” Pierre said. “Reintroducing that mask mandate would provide that additional layer of protection that will provide safety to vaccinated individuals but much more to unvaccinated individuals.”

On Monday, Steve Katsurinis, chair of the Provincetown Board of Health, said officials are monitoring the situation closely and suggested more measures could be forthcoming if the outbreak is not contained.

Officials in Provincetown said the infections were not attributable to a single super-spreader event, and at least 32 different businesses have cases associated with them, which Hanage said could indicate the presence of the Delta variant, “because of Delta’s rampant ability to be infectious and get into the next host.”

The outbreak comes as Provincetown, an LGBTQ+ summer hotspot, is at the height of its summer tourism season. Town Manager Alex Morse said Monday that the town has had two of its busiest weekends in the last two weeks. During the summer months, the town’s population balloons from 3,000 to 60,000, Morse said.

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Dr. Paul Sax, clinical director of infectious diseases at Brigham and Women’s Hospital, said he thinks it is a “reasonable assumption” that the Delta variant is fueling the outbreak. On Tuesday, the chief of the Centers for Disease Control and Prevention, Dr. Rochelle Walensky, said the variant accounts for 83 percent of cases in the United States, a dramatic uptick from the week of July 3, when Delta was responsible for 50 percent of the country’s cases.

“This kind of outbreak does give us a glimpse into what a post-opening-up society of vaccinated people will look like while the Delta variant is widely circulating,” Sax said.

Sax noted that while the vaccines provide significant protection against serious illness, death, and hospitalization, they are not 100 percent effective, and certain activities, like crowded, indoor gatherings, are associated with a higher risk of COVID-19 spread.

“I think this is the kind of thing that is starting to give us the message that COVID-19 is unfortunately here to stay,” Sax said. “It looks like we’re going to be living with it rather than getting rid of it, partially because until we have essentially everybody vaccinated, there’s going to be circulating virus, and while there’s circulating virus, there’s possibilities of transmission, even among vaccinated people.”

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“I think Provincetown should be a warning to all of us,” said Dr. Mark Siedner, an infectious disease clinician and researcher at Massachusetts General Hospital.

“The Provincetown outbreak really does illuminate the fact that we remain susceptible to outbreaks and our job is not done,” Siedner said. “The message from Provincetown should not be that vaccines don’t work, because that is not what this outbreak showed. What it showed is that even vaccinated people can become infected, especially when they’re participating in unmasked, potentially high-risk activities.”

The combination of Provincetown attracting visitors from all across the country, allowing people to gather indoors without masks, plus the Delta variant is “inviting outbreaks,” Siedner said.

“You’re saying, ‘Let’s gather together, close quarters, indoors, without masks, with a highly contagious variant, and at least some proportion of the population that’s yet unvaccinated and not protected.’ That is going to lead to outbreaks and we’ve seeing it in Provincetown,” Siedner said. “Those will continue to happen until we can find ways to get the vast majority of the population vaccinated.”

Some experts mused about how much worse the situation would have been in a community with a lower vaccination rate than Provincetown.

“If Provincetown were not vaccinated at the rate it is, they’d be having 10 times this many infections,” said Dr. Robert Duncan, an infectious disease doctor at Lahey Hospital and Medical Center.

Dr. Shira Doron, an infectious disease physician and epidemiologist at Tufts Medical Center, said Monday the cluster is “so unexpected” and “not what we’ve really seen anywhere else.”

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Doron said she is concerned the situation represents “a hole in our knowledge of the current state and what’s to come,” because “it’s not consistent with what we expect based on the data we’ve been seeing.”

In an e-mail Tuesday, the Massachusetts Department of Public Health said investigations into the vaccination statuses of the cases are ongoing and detailed how the state is helping Provincetown and Barnstable County manage the rise in cases.

The state has provided a mobile testing unit that will remain in Provincetown until at least Sunday, and is assisting Barnstable County with case investigation, cluster analysis, and contact tracing, among other efforts, the department said.


Amanda Kaufman can be reached at amanda.kaufman@globe.com. Follow her on Twitter @amandakauf1.