According to new statistics released Friday, July 30, by the Centers for Disease Control and Prevention (CDC), over three-quarters of those affected in a Massachusetts COVID-19 outbreak were fully vaccinated against the coronavirus, with four of them ending up in the hospital.

The new data, published in the Morbidity and Mortality Weekly Report of the United States, also revealed that fully vaccinated people who become infected carry as much virus in their nose as unvaccinated people and distribute it to others.

“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.”

The health agency’s Morbidity and Mortality Weekly Report found 469 cases of COVID-19 linked to multiple large public gatherings in Provincetown, Massachusetts, between July 3 and July 17.

According to the CDC, 346 cases (or 74% of all cases) occurred in people who were completely vaccinated.

Cough, headache, sore throat, myalgia, and fever were the most common symptoms among the 274 vaccinated patients.

At least five people, including four who were completely vaccinated, had to be hospitalized. There were no deaths reported.

The highly contagious delta variant was found in 90% of 133 patient specimens.

The CDC reversed course on its previous advice on Tuesday, recommending that fully vaccinated Americans living in areas with high COVID-19 infection rates wear face masks indoors again. According to a CNBC analysis, the guidelines affect almost two-thirds of the U.S. population.

“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,” CDC Director Rochelle Walensky said in a statement.

CDC also stated the data has limitations. For example, vaccinated people are expected to make up a larger proportion of COVID or CCP (Chinese Communist Party) virus cases as population-level vaccination coverage rises. Furthermore, because of detection bias, asymptomatic breakthrough infections may be underrepresented, the agency said.