Why This New Study Helped Convince the CDC to Change Its Mask Guidelines

The Centers for Disease Control and Prevention (CDC) mask guidelines for people who are fully vaccinated against COVID-19 changed last weekagain. And the results from a new, real-world CDC study looking at the effectiveness of COVID-19 vaccines were a major factor in the decision.

The new study, published in the CDC’s Morbidity and Mortality Weekly Report, includes data from 1,843 health care personnel working at 33 sites in 25 states across the U.S. Of the participants, 623 developed COVID-19 while 1,220 did not. A little more than half (55%, 340 participants) of the people who got COVID-19 were not vaccinated compared with 25% (302 people) of people who didn’t get the infection. On the other hand, 75% (918 people) of those who didn’t get COVID-19 had received at least one dose of a two-dose mRNA COVID-19 vaccine from Pfizer/BioNTech or Moderna. 

These results suggest that, in a real-world environment rather than the highly controlled clinical trials, the vaccines reduced the chances of getting symptomatic COVID-19 infections by 94%. But even getting just one dose of the vaccines provided an estimated 82% protection against symptomatic coronavirus infections in this study. 

This study did not measure the protection from vaccines against asymptomatic infections; to be considered a “case” in this study, a participant was required to have a positive COVID-19 test and at least one symptom of the infection. But other recent studies do suggest that the vaccines can help prevent asymptomatic infections as well. For instance, another CDC study published last month found that, among 3,900 participants, 205 developed COVID-19 infections, about 11% of which were asymptomatic. But the mRNA vaccines were still effective overall in this study, and they reduced participants’ risk for a coronavirus infection by about 90% once fully vaccinated.

Even if someone who is fully vaccinated does develop a COVID-19 infection (which is rare but possible), there’s a lower chance that they’d spread it to someone else than if they were unvaccinated. One of the largest studies (but not yet peer-reviewed) looking at transmission comes from Public Health England. For this study, researchers looked at data from 365,447 households with at least one person who got COVID-19. They found that, if the person who got COVID-19 was vaccinated (with either the Pfizer or AstraZeneca vaccine) at least 21 days before developing their infection, they were 40% to 50% less likely to pass the infection to the other people in their household compared with those who weren’t vaccinated.

The new CDC study provides “the most compelling information to date that COVID-19 vaccines were performing as expected in the real world,” CDC director Rochelle P. Walensky, M.D., MPH, said in a press release. “This study, added to the many studies that preceded it, was pivotal to CDC changing its recommendations for those who are fully vaccinated against COVID-19.”

In late April the CDC made its first major change to the (now familiar) face mask recommendations. With that update, people who are fully vaccinated were no longer required to wear a mask during most activities outside, except in crowded situations. But the latest guidelines allow fully vaccinated people to go without masks or social distancing in the majority of outdoor and indoor circumstances. 

Many experts said the new CDC mask guidelines are, indeed, based on solid science. And the experts pointed out that being allowed to stop wearing masks and go without social distancing could incentivize the shots for people who have yet to be vaccinated. But they also criticized the CDC for changing the guidelines so suddenly—and at a time when different groups of people have gotten vaccinated at very different rates. 

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