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How 350 vaccinated people caught COVID-19 in huge Cape Cod outbreak

Foot traffic in Provincetown, Massachusetts on July 20, 2021. A recent COVID-19 outbreak in the area has led to hundreds of infections, including many among fully vaccinated people. (Image credit: Barry Chin/The Boston Globe via Getty Images)

A large COVID-19 outbreak in a Cape Cod town has changed health officials' understanding of the coronavirus delta variant, and played a key role in the decision to recommend masks indoors even for people fully vaccinated against the virus.

The outbreak occurred in Provincetown, Massachusetts, after July 4 celebrations, and has led to nearly 900 COVID-19 infections, according to local news outlet WHDH. Health officials soon realized that many of the infected people were fully vaccinated against COVID-19. Such "breakthrough cases" were previously thought to be rare.

Now, a new report from the Centers for Disease Control and Prevention (CDC) details the outbreak investigation and gives some context behind the new masking recommendations.

Related: Coronavirus variants: Here's how the SARS-CoV-2 mutants stack up

The report describes 469 COVID-19 cases in Massachusetts residents that were tied to the Provincetown outbreak. (Most of the other cases tied to the Cape Cod outbreak are from individuals living outside of Massachusetts.)

Of these, 346, or 74%, were fully vaccinated. About 46% had received the Pfizer vaccine, 38% had received the Moderna vaccine and 16% had received the Johnson and Johnson vaccine. Most breakthrough cases, 87%, were among men, with an average age of 42. (The authors note that the demographics of these cases likely reflect those of the people who attended the area's events, which "were marketed to adult male participants.")

About 80% of breakthrough cases experienced symptoms of COVID-19, including cough, headache, sore throat, muscle aches and fever, the report said. A total of five people were hospitalized, four of whom were fully vaccinated. Two of the four fully vaccinated patients had underlying medical conditions, the report said.

When health officials analyzed swab samples from 133 cases, they found that nearly 90% were infected with the delta variant. And when they analyzed samples from 211 cases, they found that the so-called CT value,  a proxy value for viral load, was about the same among fully vaccinated people as those who were not vaccinated or partially vaccinated.

"High viral loads suggest an increased risk of transmission and raised concern that, unlike with other variants, vaccinated people infected with delta can transmit the virus," Dr. Rochelle Walensky, director of the CDC, said in a statement about the findings released Friday (July 30). "This finding is concerning and was a pivotal discovery leading to CDC's updated mask recommendation."

The agency's updated mask guidance, released on Tuesday (July 27), recommended that people who live in areas with "high or substantial" COVID-19 transmission wear masks indoors, even if they are fully vaccinated. But the authors of the new report said their findings suggest that recommendations might go further, to advise universal masking.

"Even jurisdictions without substantial or high COVID-19 transmission might consider expanding prevention strategies, including masking in indoor public settings regardless of vaccination status, given the potential risk of infection during attendance at large public gatherings," the report said.

Originally published on Live Science.  

Rachael Rettner

Rachael has been with Live Science since 2010. She has a master's degree in journalism from New York University's Science, Health and Environmental Reporting Program. She also holds a B.S. in molecular biology and an M.S. in biology from the University of California, San Diego. Her work has appeared in Scienceline, The Washington Post and Scientific American.