So you got the vaccine. Can you still infect people? Pfizer is trying to find out.

Sebastián De Toma joined Pfizer’s clinical trial last year, getting his shots in August and September. The Argentinian journalist still doesn’t know if he got the real covid-19 vaccine or the placebo, but on Sunday, January 31, the trial doctors called him with a new offer.

Would De Toma be willing to undergo a series of nasal swabs to regularly test for the virus? He says the doctors offered to send Cabify (a Spanish ride-sharing service) to bring him to the Hospital Militar in Buenos Aires. “They’ll swab me on the go, through the car window, and that’s it,” says De Toma.

The extra coronavirus tests, being offered to some volunteers in Argentina and in the US, are part of a plan by Pfizer to help answer a key covid unknown—how often vaccinated people develop asymptomatic coronavirus infections and whether they can still spread the virus, despite getting the shot.

Whether or not the vaccines stop “onward transmission” of the virus is likely to be a critical variable in determining how the pandemic plays out and how soon life goes back to normal. Right now, researchers say, their best guess is that vaccines will reduce transmission but may not prevent it entirely.

“We don’t know, but it’s an important question because the answer will influence mask wearing; it will influence behavior; it relates to comfort going to restaurants and movies and the overall benefit we can expect with vaccines,” says Lawrence Corey, who leads operations for the Covid-19 Prevention Network, which carried out several US vaccine trials.

The silent spreader mystery

“There are three things a vaccine can do: stop you from acquiring the disease altogether, stop onward transmission, and stop symptoms,” says Jeffrey Shaman, a public health researcher at Columbia University. A perfect vaccine would create what is called “sterilizing” immunity, which means the virus can’t get a foothold in your body at all. Some inoculations, however, do allow low-level infections that people’s immune systems fight off without any symptoms. Their bodies still accumulate a certain quantity of the virus, which they may be able to transmit to others.

The reason we don’t know how well vaccines stop this transmission is that it’s expensive and complicated to measure. When companies like Pfizer, Novavax, Moderna Therapeutics, and others launched big studies of their new covid-19 vaccines last year, they were testing whether the vaccines could prevent people who caught the disease from getting sick or dying. The results on that count were impressive: hardly anyone who is vaccinated ends up in an ICU on a respirator.

What they didn’t measure was the “indirect” effect of vaccines in preventing the further spread of the virus, even though some computer models have predicted that blocking transmission could save more lives. One model, published in August by a team at Emory University, found that a vaccine that’s good at stopping spread, but not very good at stopping disease, would still lead to fewer deaths overall because it would slow the outbreak enough to reduce the total number of people who get infected.

A step toward understanding how often vaccinated people spread the virus is what Pfizer is doing now: trying to figure out whether people like De Toma are getting infected without ever feeling sick.

The evidence so far suggests that vaccines should cut the chance of transmission, but may not eliminate it. For example, vaccinated monkeys spritzed with the virus do get infected but don’t become particularly sick. Overall, they have much less virus in their airways. “There is strong evidence that contagiousness is correlated with symptoms. If you can cut down symptoms, you are probably cutting down transmission,” says Shaman.

But that doesn’t mean there is no spread. Early in the pandemic, researchers discovered that some people who caught the coronavirus and never felt sick were still spreading the disease. The evidence now suggests that the role of such “silent spreaders” is substantial, even though they tend to infect fewer people on average. In a report published January 7,  a team including epidemiologists at the Centers for Disease Control and Prevention estimated that a third of people infected with the coronavirus never develop symptoms and that they cause about a quarter of all spread.

Moderna Therapeutics, maker of another vaccine, did not reply to questions about whether it is studying transmission. However, preliminary data the company submitted to the US Food and Drug Administration in December offered one clue: people who got one dose of the vaccine were 66% less likely to turn up positive on a coronavirus test than those who got the placebo. Moderna suggested that “that some asymptomatic infections start to be prevented after the first dose.”

While looking for the virus in people’s noses can detect silent infections, it doesn’t actually prove whether these people can then infect others. To figure that out, researchers at the Covid-19 Prevention Network last year proposed studying more than 20,000 students on two dozen US campuses, including Louisiana State University. They proposed “almost daily” nose swabs to monitor exactly when the virus appeared, and in what amounts, in the airways of both vaccinated and unvaccinated students. Then, with contact tracing, they hoped to map how often vaccinated students spread the virus.

“You can learn a lot by understanding the acquisition and viral titers in the nose,” says Corey. “Then close contact tracing could estimate how often the people spread the virus, which is known as forward transmission.”

On December 31, the Wall Street Journal reported that the proposed study had failed to win funding, because of high costs and questions over its feasibility. Corey says the group has since updated the proposal and that it again being considered by the National Institutes of Health. He believes the study is worth the effort. “We just need to know,” he says. “Because we may need to turn our attention to the kinds of vaccines that do reduce transmission.”

Stemming the flood

Researchers know that stopping transmission is the only way to get rid of the coronavirus for good. One way the pandemic can end is via “herd immunity”—that is, when enough people are vaccinated, or infected, for the outbreak to recede on its own because there aren’t enough people left to infect. That threshold is commonly thought to be about 70% of the population.

But if vaccinated people are “leaky”—if they can still spread the virus sometimes—the threshold will rise. In fact, according to basic outbreak math, if the vaccine stops anything less than two-thirds of transmission events, it’s impossible to reach herd immunity at all. And that’s not even considering that many people will refuse the vaccine, nor mounting evidence that immunity may not last against new variants of the virus.

So if the vaccines don’t almost completely stop transmission, “you will still have continued circulation and there won’t be much herd immunity,” Corey says. “It’s going to be in the population a long time.”

Jody Lanard, a medical risk communicator who has worked with the World Health Organization, says until questions about vaccine transmission are answered, public health officials will likely send out contradictory messages. On the one hand, she says, exhorting people to “keep wearing a mask” implies that a vaccinated person can still transmit the virus. At the same time, encouraging everyone to get vaccinated, even those who are not in a high-risk group, “leans heavily on the notion that transmission will likely be reduced by vaccination.”

Lanard herself took extreme measures to avoid the virus, barely venturing out; when she did, she wore special masks and goggles in her building’s elevator. She says she recently managed to get a vaccine appointment, and now that she’s had the shot, she thinks she may loosen up and visit with some younger relatives.

But Lanard says she is still going to wear a mask at least until case numbers in New York, where she lives, go back down. “It would be so stupid to catch covid at this point in the pandemic,” she says. “I’m a fairly well-protected grandmom now, thanks to the vaccine. The last thing I want to do is infect some unprotected grandmom.”