Roughly three-fourths of people who were infected in a COVID-19 outbreak in Massachusetts were fully vaccinated against the virus, according to new data published Friday by the Centers for Disease Control and Prevention (CDC).

Key findings from this outbreak led to the CDC’s updated mask guidance, which was announced this week.

What You Need To Know

  • Roughly three-fourths of people who were infected in a COVID-19 outbreak in Massachusetts were fully vaccinated against the virus, according to new data published Friday by the CDC

  • The study, which examined 469 COVID-19 cases linked to a July outbreak in Barnstable County, Mass., found that 74% of the cases were breakthrough infections among fully-vaccinated individuals

  • Of the fully-vaccinated individuals who contracted breakthrough infections, 79% were symptomatic

  • The finding that vaccinated people carried similar amounts of the virus as those who are unvaccinated helped lead to the CDC's updated mask guidance which was announced earlier this week

The data, published Friday in the CDC’s Morbidity and Mortality Weekly Report (MMWR), examined a July outbreak in Barnstable County, Mass., which includes popular vacation destination Provincetown.

Of 469 positive COVID-19 cases, approximately three-quarters — 74%, or 346 cases — were so-called "breakthrough infections" among fully-vaccinated people who had either received the two-dose mRNA vaccines (Pfizer-BioNTech or Moderna) or one dose of the single-shot Johnson & Johnson vaccine.

Seventy-nine percent of that group — 274 people — experienced symptoms, the most common of which being a cough, headache, sore throat, myalgia and fever.

Among five people who were hospitalized, four were fully vaccinated.

Testing found that the delta variant was largely to blame, identifying the variant in 90% of specimens from 133 patients, per the report.

Most significantly, the CDC study found that vaccinated people carried similar amounts of the virus as those who are unvaccinated, suggesting that fully-vaccinated persons can still spread the virus.

“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 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 new report comes just days after the CDC reversed course its federal mask guidance earlier this week — recommending masks be worn indoors by all persons in high-risk areas, regardless of vaccination status. The CDC also recommended Tuesday that masks be worn by all K-12 students when they return to classrooms this fall.

Researchers said the data shows that while fully-vaccinated people remain protected from severe infection in most cases, they could still pose a risk to others.

“Findings from this investigation suggest that 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 that include travelers from many areas with differing levels of transmission,” the report reads.