Covid-19 Vaccines Make Breakthrough Infections Shorter, Less Contagious: CDC Study

New research published this week is the latest to indicate that vaccinated people have much less to fear from covid-19. The study found that even in the rare cases when people given mRNA vaccines developed infections, they tended to experience shorter illness and produce less of the virus than unvaccinated people—both important factors when it comes to the risk of transmitting it to others.

The research was conducted by scientists from the Centers for Disease Control and Prevention’s covid-19 response team and was published Wednesday in the New England Journal of Medicine. The team analyzed data from two ongoing surveillance studies of healthcare and front-line workers in six states, collected between December 14, 2020 and April 10, 2021. As part of these studies, the workers were tested weekly for the coronavirus, as well as when they developed symptoms suspected to be caused by covid-19. The team specifically focused on the nearly 4,000 workers who hadn’t yet tested positive by mid-December.

By April, about 80% of these workers had gotten either of the two mRNA vaccines developed by Pfizer/BioNTech and Moderna, with 80% of them having been fully vaccinated. During the entire study period, 205 workers contracted covid-19. Only 5% of these cases were among the fully vaccinated, indicating that the vaccines were 91% effective against infection, which mirrors the real-world success seen in other studies (another 11% of cases were among the partially vaccinated). And when the team studied these breakthrough infections, as they’re called, they found a clear difference between them and cases found in unprotected people.

Compared to infections in unvaccinated people, people with breakthrough infections had lower viral loads (the amount of detectable viral DNA/RNA measured by a test) and shorter time windows where any virus was detected at all. About 72% of the unvaccinated infected people had detectable viral RNA for more than a week, for instance, while about 75% of the partially or fully vaccinated infected people had detectable coronavirus in their system for only a week. Perhaps best of all, vaccinated people also were less likely to develop common symptoms like fever and had shorter-lasting symptoms when they did. They spent about six fewer days with symptoms on average, and spent two fewer days sick enough to be in bed.

The study does have limitations. For one, though most vaccinated Americans have received an mRNA vaccine, there are around 12 million who have gotten the Johnson & Johnson one-dose shot instead, which is thought to be less effective at preventing infection (though similarly effective in preventing serious illness). The study was also conducted before the arrival of the Delta variant, a more transmissible strain of the virus that will soon be dominant in the U.S. and elsewhere. Research has suggested that both mRNA vaccines provide very similar effectiveness against Delta with the full two doses.

This isn’t the first research to suggest that vaccinated people who become infected are still better off than those who forgo the shot. But it appears to be the largest prospective study of its kind (prospective studies track people in real time, allowing for clearer evidence of a cause-and-effect relationship). And should other data continue to show similar results, the CDC authors say it’s good news for the hope that these vaccines will both directly protect people from covid-19 and help keep them from spreading the virus to others if they do catch it.

It’s a finding that would be “especially important to essential and frontline workers, given their potential to transmit the virus through frequent close contact with patients, coworkers, and the public,” the researchers wrote.