CDC study shows three-fourths of people infected in Massachusetts covid-19 outbreak were vaccinated

A sobering scientific analysis published Friday of an explosive Massachusetts covid-19 outbreak fueled by the delta variant found that three-quarters of the people who became infected were fully vaccinated. The report, from the Centers for Disease Control and Prevention, is one key piece of a growing body of evidence that bolsters the hypothesis that vaccinated people can spread the more transmissible variant and may be a factor in the summer surge of infections.

Critically, the study found that vaccinated individuals carried as much virus in their noses as unvaccinated individuals, and that vaccinated people could spread the virus to each other. The CDC was criticized this week for changing its mask guidance without citing unpublished data. The report released Friday contains that data.

“This finding is concerning and was a pivotal discovery leading to CDC’s updated mask recommendation,” CDC director 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.”

Scientists said the Provincetown outbreak and other recent data on breakthrough infections make clear that the vaccines offer significant protection, as they were designed to, against severe illness and death but do not offer blanket protection against any chance of infection. Only a handful of people in the outbreak were hospitalized, but four of them were fully vaccinated.

A CDC internal document obtained by The Washington Post estimated that 35,000 vaccinated people a week in the United States are having symptomatic breakthrough infections out of a vaccinated population of more than 162 million. Vaccination coverage is higher than average in Massachusetts, with nearly 70 percent of residents fully vaccinated.

“This shows the delta is formidable,” said Larry Corey, a virologist at the Fred Hutchinson Cancer Research Center. “We can’t take one report of packed bars and extrapolate and say the sky is falling. The sky is not falling. But it does say the vaccine is not infallible.”

“Common sense has to be used,” Corey said. “It’s a learning moment, it’s a teaching moment. You can’t overlook the vast data we have on the effectiveness of the vaccine.”

Corey and other scientists reiterated that vaccination was the best way to end the pandemic. Turning covid-19 infections from potentially fatal pneumonia to nuisance colds was the main goal of the shots.

“People should be reassured that if they are fully vaccinated, that they are very likely, highly likely, to be protected against severe or critical illness, the kind of illness that would cause them to be hospitalized or killed by this virus,” said Paul A. Offit, a vaccine expert at Children’s Hospital of Philadelphia. “Vaccines save your life.”