The COVID-19 vaccines in use now have been touted for how well they work—the Pfizer vaccine trials found its product to be 95 percent effective in preventing COVID-19 infections after both shots and the Moderna 94.1 percent effective after the same. And people are gradually becoming familiar and comfortable with how the mRNA technology that allowed these to be developed so quickly works. Scientists now have answers to many early questions about the vaccines.

But a question that there’s still no solid answer to is whether people who’ve been vaccinated can still carry the virus and potentially spread it to others.

The question arose as early as December, when Pfizer CEO Albert Bourla appeared on Dateline NBC” alongside the CEOs of Moderna and Johnson & Johnson. "I think this is something that needs to be examined,” Bourla told Dateline. “We are not certain about that right now."

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“There is a theoretical risk that you could pass the virus on to others despite being vaccinated,” says Kirsten Hokeness, Ph.D., director of Bryant University’s new Center for Health and Behavioral Sciences. And in March 2021, scientists began research that will help answer the question of whether this is an actual risk or not. Called the PreventCovidU study, it's recruiting 12,000 college students to get the Moderna Covid vaccine, take daily nose swabs for about four months, and also get regular Covid tests. This will help determine whether vaccinated people can pass along the virus to others, including whether people who've been vaccinated, have Covid, but are asymptomatic, can pass the virus along to others. Researchers say results should be in in time for the fall return to school.

Virus transmission by vaccinated people hopefully won’t be the case. “The goal of the vaccine is to create immunological memory in the body so that when you encounter the virus in the future, your immune system rapidly ramps up and attacks the virus very quickly before you get sick. Therefore, as long as the vaccine boasts a strong immunological response, it is likely that the virus will be stopped from replicating in your system pretty quickly,” Dr. Hokeness says. That would limit your ability to spread it to others. “It can happen, but the risks would be far less than if you were not vaccinated,” she says, adding that “since the vaccine requires two doses, it may be that your ability to spread the virus would be greater if you only got one dose, but there is not data available yet that would suggest that that is the case."

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Meanwhile, health experts are encouraging people who are vaccinated to act as if they could somehow pass virus they had acquired somewhere to other people—that means they need to keep masking and social distancing (and you do, too).

While vaccines are a vital tool in combating the pandemic, having them doesn’t mean everyone can safely return to “normal” right away. Or forever—it's not clear exactly how long post-vaccination immunity would last. That means keep doing what you’re doing, even after you receive the vaccine, until more research comes out.

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Marty Munson

Marty Munson, currently the health director of Men's Health, has been a health editor at properties including Marie Claire, Prevention, Shape and RealAge. She's also certified as a swim and triathlon coach.

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Jesse Hicks is a Detroit-based writer and former features editor at The Verge who specializes in longform stories about science, health, and technology. He has written for Men’s Health, VICE, Harper’s, and many other publications.