Chatham, Cape Cod

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Nearly three-fourths of the infections in a recent Massachusetts COVID-19 outbreak were breakthrough cases.

The Centers for Disease Control and Prevention released a study on Friday that found 346 of the 469 reported coronavirus infections occurred in people who were fully vaccinated. The outbreak happened between July 3–17 in Barnstable County as large summer gatherings took place in Provincetown.

Of the breakthrough cases, 274 were symptomatic, while just four people were hospitalized. One other infected who had not been vaccinated was also hospitalized.

Testing found that 90% of specimens from 133 patients contained the Delta variant. The most common side effects were cough, headache, sore throat, muscle pain, and fever. No deaths were reported.

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

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Getty A person getting vaccinated

The CDC previously released new guidelines on Tuesday based on new science around the Delta variant, which has become the most predominant strain circulating in the United States. Appearing in four out of five infected samples, the Delta variant has been known to infect vaccinated people and spread to others on occasion.

The new guidelines included the continued push for COVID-19 vaccinations, as they have been shown to reduce the spread of COVID-19, in addition to preventing severe illness, hospitalization, and death, even with the Delta variant.

Vaccinated and unvaccinated people in areas with substantial-to-high transmission rates are recommended to wear masks in public indoor spaces. Additionally, those who are not inoculated are urged to continue wearing masks until they are fully vaccinated.

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Meanwhile, the CDC recommends K-12 students return to full-time in-person learning in the fall, with proper COVID precautions in place.

“This moment, and most importantly, the associated illness, suffering and death, could have been avoided with higher vaccination coverage in this country,” said Walensky. “COVID-19 continues to present many challenges and has exacted a tremendous toll on our nation. We continue to follow the science closely and update the guidance, should the science shift again. We must take every step we can to stop the Delta variant and end this pandemic.”

As information about the coronavirus pandemic rapidly changes, PEOPLE is committed to providing the most recent data in our coverage. Some of the information in this story may have changed after publication. For the latest on COVID-19, readers are encouraged to use online resources from CDC, WHO, and local public health departments and visit our coronavirus hub.