A federal investigation into a COVID-19 outbreak earlier this month in a Massachusetts county that’s home to Cape Cod found that 74% of the 469 infections were among vaccinated people, a finding that raises questions about the prevalence of breakthrough infections.

The research, published Friday afternoon by the Centers for Disease Control and Prevention, is the latest sign that people who are fully vaccinated may need to return to mask wearing, social distancing and other mitigation measures as the more infectious delta variant tears through the U.S.

Earlier this week, the CDC shifted its stance and began calling for people, even those who are vaccinated, to again wear masks in public indoor settings in areas of the country with “high” or “substantial” levels of community transmission.

The public health agency also recommended that all K-12 students and educators wear masks indoors. At the time, little data was provided to the public to back up its position.

However, it seems that the CDC is increasingly worried about the virus’s ability to move with travelers and throughout crowded gatherings, even in lower-risk areas of the U.S.

“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 authors wrote.

The CDC described several large public gatherings that brought people from around the U.S. to Barnstable County in Massachusetts from July 3 to 17. People began testing positive for COVID-19 around July 6, and many cited attendance at crowded indoor and outdoor bars, restaurants and homes, notably in Provincetown, at the farthest reach of the Cape Cod peninsula.

Here’s what we know about the breakthrough infections:

• 274, or 79%, of the fully vaccinated people who tested positive for the virus were symptomatic.

• Out of the 133 specimens that were sequenced in this outbreak, 119, or 89%, were from the delta variant, and one sample had the delta AY.3 sublineage.

• Four of the five people who were hospitalized in this outbreak were vaccinated. No one died.

• 301, or 87%, of the people who had been vaccinated and tested positive were men; their median age was 42.

• Of the people who reported breakthrough infections, 159, or 46%, had gotten the BioNTech SE

–Pfizer Inc.

vaccine; 131, or 38%, had received the Moderna Inc.

shot; and 56, or 16%, had been administered the Johnson & Johnson


Earlier this week, the CDC director, Dr. Rochelle Walensky, also said that new data had revealed that people who are vaccinated and test positive for the virus can carry the same viral load as people who are unvaccinated and test positive for the virus. This research found that cycle threshold (Ct) values in samples from 127 fully vaccinated people and 84 unvaccinated people were similar.

“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,” Walensky said in a separate statement on Friday. “This finding is concerning and was a pivotal discovery leading to CDC’s updated mask recommendation.”

There is little available data so far about breakthrough infections, which are still considered to be rare among the 49.5% Americans who have been fully vaccinated. In instances when they do occur, hospitalization or death is even more rare.

Back in May, the CDC said that there had been fewer than 10,000 breakthrough cases among the 95 million or so Americans who were fully vaccinated at that time.

However, there is data emerging that points to higher rates of breakthrough infections than previously thought, although these types of cases are still considered rare.

Los Angeles County has found that in June one-fifth of all new cases were breakthrough infections among the vaccinated, and a study published this week in the New England Journal of Medicine discovered a breakthrough infection rate of 2.6% among about 11,000 vaccinated health care workers at Sheba Medical Center, which is located near Tel Aviv.

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