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Scientists studying the massive COVID outbreak in Provincetown made a startling discovery that fueled the US Centers for Disease Control and Prevention’s new masking policy this week: that vaccinated people who become infected can carry as much of the virus as unvaccinated patients.

The CDC findings suggest that vaccinated people could just as easily spread the virus to others. Most of the infections are of the Delta variant, and in Provincetown, where the cluster is now approaching 900 cases, authorities said that three-quarters of the infected people had been vaccinated.

The ominous finding was contained in a new report the CDC published Friday that provided the scientific backdrop to its decision to recommend Americans living in high transmission areas — which now includes much of the country — wear masks in indoor public spaces, even if they are already vaccinated.

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“This finding is concerning and was a pivotal discovery leading to CDC’s updated mask recommendation,” Dr. Rochelle Walensky, CDC director, said Friday. “The masking recommendation was updated to ensure the vaccinated public would not unknowingly transmit virus to others, including their unvaccinated or immunocompromised loved ones.”

The agency added that the Provincetown cluster suggests local governments may want to go further — even in places like Massachusetts where the virus isn’t spreading as rapidly. Earlier this week, officials in Provincetown imposed mandatory mask-wearing for indoor public spaces.

“A lot of people have gone back to business as usual, and this just shows we can’t do that,” said Dr. Pardis Sabeti, a leader in Harvard and MIT’s Broad Institute’s infectious disease program, which conducted genetic sequencing of the samples. “This is unfortunate, but we’re going to see a lot of these popping up: The Delta variant is highly transmissible and can infect vaccinated people.”

Sabeti said that vaccinated people should remember they are well-protected from severe illness; out of 882 infections, most experienced either no symptoms or flu-like symptoms; seven went to a hospital; no deaths have been reported.

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But Sabeti said that outbreaks such as these pose the gravest danger to people who are at higher risk of serious illness, including those who aren’t vaccinated, such as children under 12, and could raise the likelihood of a future mutation that eventually defeats current vaccines.

“The more mutations that happen, the more shots on goal it has to get around these things,” said Daniel Park, a senior group leader of viral computational genomics at the Broad Institute.

According to the CDC’s report, 469 COVID-19 cases were identified among Massachusetts residents who had traveled to Provincetown between July 3 and July 17, including 346 fully vaccinated people. About 274 of the vaccinated people with so-called breakthrough infections showed symptoms, most commonly cough, headache, sore throat, muscle pain, and fever.

Testing identified the highly contagious Delta variant in 90 percent of specimens from 133 patients. Park at the Broad Institute said he and his colleagues were actually relieved to learn the outbreak didn’t represent a new variant.

Public health authorities have continually emphasized that breakthrough infections are rare, and that vaccinated people who contract the virus are unlikely to become severely ill.

But the findings provided an important new clue on the Delta variant: that vaccinated people may spread it just as easily as unvaccinated people through sneezing or coughing. Park said researchers are still working to confirm whether that’s true.

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Provincetown has one of the state’s highest vaccination rates, with virtually all eligible residents being fully vaccinated, state data show. And as a legendary summer destination for LGBTQ people, it may have also benefited from an awareness of public health that not all populations share.

Even though many of those infected were vaccinated, they still sought COVID tests upon feeling ill. State Senator Julian Cyr, who represents the Outer Cape, said LGBTQ people are hyper-aware of the importance of regular medical tests following the HIV/AIDS epidemic.

“I’ve grown up being told to go get tested — there’s a real culture of health care-seeking behavior,” said Cyr, who is gay. He believes that’s “one of the reasons why we picked up on this so early in Provincetown.”

But even though most infections have not led to severe illness, the outbreak has shown that the virus can still raise alarm — and thwart people’s summer plans — even in places with high vaccination rates.

After a jubilant June in which newly vaccinated tourists flocked to Provincetown to party, dance, and watch shows, the mood clearly dampened about a week after Fourth of July festivities. Some vaccinated people started posting on social media about contracting COVID, said Radu Luca, executive director of the Provincetown Chamber of Commerce.

That scared many in town who had been acting as though they were back to their pre-COVID lives, Luca said. More people began getting tests, as the state recognized the burgeoning cluster and dispatched a mobile unit that has now tested more than 5,000 people, Cyr said.

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On July 19, the town “strongly advised” venues that couldn’t ensure social distancing to require vaccination records for entrants. Many now do.

Now, the town is seeing some hotel customers cancel their bookings. Fewer people are making day trips from the rest of the Cape or from Boston via the ferry, Luca said.

“The Delta variant is everywhere,” Luca said, adding that vaccinated people need to learn to be cautious but continue living life. “We’ve learned to live with the flu and some people who get the flu get mild symptoms and some people get severe symptoms and end up in the hospital. The vaccine is doing exactly what it was supposed to do.”

CDC officials thanked the Provincetown community for enthusiastically participating in the investigation. That helped health officials move faster, said Dr. Celine Gounder, an associate professor of medicine and infectious diseases at New York University’s Grossman School of Medicine.

“You’re going to capture more infections if the people involved are more forthcoming” in answering contact tracers’ questions, Gounder said.

But what’s happened in Provincetown — and the evidence of high viral loads in vaccinated people — has already changed the national conversation around COVID-19.

Officials reported cases among residents of 22 states who traveled to Provincetown between July 3 and July 17, as well as secondary transmission cases.

And in light of the CDC’s new recommendations, the Massachusetts Department of Public Health on Friday advised fully vaccinated residents to mask up in indoor spaces outside their homes if they or someone they live with are at high risk for complications from the virus.

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In Provincetown, the outbreak appears to be easing. Town manager Alex Morse wrote on Facebook that the rate of positive tests has declined from a peak of 15 percent on July 15 to a new low of 4.6 percent on Thursday. The cluster will be considered contained if the rate drops below 1 percent.

On Friday, Morse reported that there are 103 active cases in town, and that 170 other people who had tested positive are now in the clear.

Dr. Robert Horsburgh, a Boston University professor of epidemiology, said Friday that the breakthrough cases in Provincetown haven’t come as a surprise. It could have been much worse if fewer people in Provincetown had been vaccinated, he said.

“This is what we expect in a highly vaccinated population,” Horsburgh wrote. “Even though the COVID vaccines are very effective, they are not perfect.”


Travis Andersen can be reached at travis.andersen@globe.com. Follow him on Twitter @TAGlobe. Naomi Martin can be reached at naomi.martin@globe.com.