Your questions, answered "Are vaccinated people who have breakthrough Covid less contagious than unvaccinated people who have Covid? Are there any studies comparing the transmission rates? People I know who have had breakthrough Covid have not spread it to their spouses, children, co-workers. Why is that?" — Meredith in Illinois This question has been the subject of months of intense debate. We know that breakthrough infections, while uncommon, can spread the virus. Scientists are still trying to gauge the transmission risk. But a growing body of research indicates that vaccinated people are less likely than unvaccinated people to infect those around them. In other words, yes, the vaccinated do appear to be less contagious than the unvaccinated — or at least contagious for a shorter period of time. "It's a somewhat muddled and complicated picture," Timothy Brewer, an epidemiology professor at the University of California at Los Angeles, told me. "But at least at a population level, there are data to show that the contacts of vaccinated individuals who become infected are less likely to get infected themselves." One of the benefits of the vaccines is their ability to prevent you from getting infected at all. They're not perfect — their main purpose is to stave off severe illness — but they do this quite well. If you're not carrying the virus, you can't spread it. Even if you get a breakthrough case, several studies this year suggest you're less likely to pass the virus onto others if you've had the shots. Researchers in Britain found that the likelihood of household transmission was 40 to 50 percent lower in the homes of vaccinated people compared with unvaccinated people. Another non-peer-reviewed study out of the Netherlands found that transmission to unvaccinated housemates was 63 percent lower among vaccinated people. Yet another study, also not yet peer reviewed, examined hundreds of thousands of cases and concluded that people were less likely to infect their close contacts if they were vaccinated, though it found that this protection diminished about three months after their second dose. "Not only does getting vaccinated reduce your risk of becoming seriously ill from the virus, it also protects those around you," Tom Frieden, a former CDC director, said of the research. It's possible that vaccinated people who develop breakthrough cases may carry similar amounts of the virus as unvaccinated people at the height of infection. Some studies indicate that this "peak viral load" can be about the same for both groups. But there's compelling evidence that vaccinated people get rid of the virus faster. "The reason that vaccinated people are less likely to transmit the virus is because their immune system has been trained to recognize and respond quickly to the virus and is able to limit both how much virus they produce and period of time they produce it for," said Christopher Brooke, a microbiologist and co-author of a paper showing virus shedding and covid-19 symptoms were lower in vaccinated people. "By reducing the amount of virus shed and the duration of virus shedding," he told me, "vaccination reduces the odds that someone will transmit." None of that is to say that vaccinated people should take their foot off the brake if they develop symptoms or test positive. It's just as important for people who've gotten the shots to stay home if they're sick and isolate until their infection clears. "Vaccines do reduce transmission. They don't eliminate it though. That's an important caveat," said Brewer, of UCLA. "But you're clearly better off being vaccinated than not, or being around vaccinated individuals than not." | 
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