Here’s the Study That Convinced the CDC to Recommend Vaccinated People Wear Masks Again

This week, the Centers for Disease Control and Prevention (CDC) reversed course and said that vaccinated people should wear masks in some circumstances. Now the CDC is releasing the study that convinced experts to make the change—especially with the delta variant circulating so widely.

The new study, published in the CDC’s Morbidity and Mortality Weekly Report, centers on a cluster of COVID-19 infections, including breakthrough infections among fully vaccinated people, in Provincetown in Barnstable County, Massachusetts. Following a series of large public events that attracted tourists from outside the county, researchers identified 469 cases of COVID-19. Nearly three-quarters of those cases (346) occurred in people who were vaccinated. And of the five people who required hospitalization, four were fully vaccinated. There were no deaths recorded in the study.

By now it is probably no surprise that the delta coronavirus variant, which is known to be highly transmissible, was responsible for the majority of those cases: Researchers sequenced 133 cases and tied 90% of them to the delta variant.

The really concerning part here is that the researchers measured the amount of virus present in people’s upper airways—and found that fully vaccinated people had about the same amount as unvaccinated people. That, as well as the breakthrough infection data, suggests that fully vaccinated people can not only develop COVID-19 infections (thanks largely to delta) but also that they can spread those infections to other people. With this information, it “makes sense to recommend masking in high-risk settings for all until we learn more,” Abraar Karan, M.D., MPH, an infectious disease fellow at Stanford University, wrote on Twitter.

“The delta variant is different. It is more infectious than any other variant to date,” Celine Gounder, M.D., clinical assistant professor of medicine and infectious diseases at New York University’s Grossman School of Medicine, said on Twitter. “When things change, we’ve got to adapt.”

However, it’s important to resist the urge to make conclusions about the overall effectiveness of the vaccines against delta based on these results, the authors say, even with the relatively large share of COVID-19 cases in this study that occurred among vaccinated people. Massachusetts has one of the highest vaccination rates in the country (more than 60% of the population), so it makes sense that they would represent more of the share of cases. And we’re working with an incomplete set of data here. We don’t know how many more people would have gotten sick or hospitalized if they hadn’t been vaccinated, for instance.

Also, when looking at the number of breakthrough cases per vaccine received, it mostly lines up with the number of people who received that vaccine in the general population: 46% of vaccinated cases in the study received the Pfizer/BioNTech vaccine (compared to 56% in the general population), 38% got the Moderna vaccine (compared to 38% in the general population), and 16% got the Johnson and Johnson/Janssen vaccine (with 7% in the general population). So, again, this does not necessarily suggest that one vaccine did better against delta than any other vaccine.

“The vaccines are largely working as expected,” Ashish K. Jha, M.D., MPH, dean of the Brown University School of Public Health, said on Twitter. “We had an outbreak in P-town with lots of vaccinated folks infected. No one died. Very few got sick, and things have returned to normal. This is how vaccines work, folks.”

The take-home message, the study authors say, is that their results indicate that masks and other public health measures—for vaccinated and unvaccinated people—are a good idea in high-risk situations. Even in areas without substantial or high coronavirus transmission rates, wearing a mask indoors can help prevent the spread of the virus regardless of your vaccination status.

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