Do Vaccines Stop the Spread of COVID-19? What You Need to Know

The way most people think of vaccines is pretty simple: you get vaccinated, and your immune system is primed and trained to fight off the invisible intruder in question, be it virus or bacteria. If you’re protected, you can’t be infected, and if you’re not infected, then you can’t spread it to anyone else.

And that’s true most of the time. But not all vaccines work that way, and it’s not actually what the two COVID-19 vaccines authorized by the U.S. Food and Drug Administration—made by Moderna and Pfizer-BioNtech—are designed to do. Their effectiveness is measured by how well they protect people against moderate to severe COVID-19 disease—not how well they prevent infection or spread of the COVID-19 virus itself.

That’s an important thing to remember as more doses reach more people in coming weeks. While the shots are 94% to 95% effective in preventing disease, there aren’t definitive data yet that they completely shut down the virus enough to stop it from moving from an infected person to someone else—say, if that vaccinated but infected person isn’t wearing a mask or isn’t keeping the recommended six feet apart from others. That’s why public health officials have said that even if you’re vaccinated, you can’t, unfortunately, ditch the mask or start holding intimate dinner parties with your other vaccinated friends and family. That won’t happen until researchers finish their studies on the vaccines, which will take another several months, to determine how well the shots have stopped the virus from jumping ship from a vaccinated person to someone else.

However, there has been some early good news on that front, thanks to two recent studies, one by AstraZeneca and one by a group of Israeli scientists. Both groups recently published data suggesting that COVID-19 vaccines might reduce the spread of virus, but that data begs closer scrutiny. (AstraZeneca’s vaccine is authorized in the U.K. but the company has not yet applied for authorization in the U.S.)

On Feb. 1, AstraZeneca and its academic partners at University of Oxford reported the results of their late-stage, Phase 3 studies of their vaccine. It’s made from a weakened adenovirus that causes infections in chimps, which was modified to carry the genetic instructions for making a COVID-19 viral protein. This protein triggers the human immune system to make antibodies against it, which, if the vaccinated person is then infected with the virus, can block the actual COVID-19 virus from sticking to and infecting cells. In the study, the scientists said that their shot was 67% effective in protecting people from COVID-19 disease, and nearly 100% effective in shielding them from severe disease requiring hospitalization.

They also took weekly swabs from the study volunteers and tested them for the COVID-19 virus, and found 50% fewer positive tests among people who were vaccinated than among people who weren’t. Because people who don’t test positive are less likely to be infected, and therefore unable to spread the virus, they extrapolated from that data that their shot can lower spread of the virus.

That might be the case, but public health officials say the findings still need to be confirmed. Yes, the people who were vaccinated had less virus in them if they were infected, but that doesn’t necessarily mean that they aren’t still able to spread the virus to others. As infectious disease experts like to say, all it takes is one virus to cause an infection. “The fact is that what they showed is that there was decreased viral shedding, or decreased detection of the virus,” says Dr. Carlos del Rio, an infectious disease expert and executive associate dean at Emory University. That doesn’t mean, he says, that the virus isn’t still there and can’t be transmitted to another person.

A week later, scientists in Israel published a preliminary report hinting that among the 20% or so of its population who have been vaccinated, mostly with the Pfizer-BioNTech shot, those who were vaccinated might be less likely to spread the virus. In that study, the researchers compared people over age 60, who were among the first in the country to get vaccinated, beginning in late December, to those aged 40-60, who have recently been invited to get immunized. Since the shot requires two doses, most of the first vaccinees in the older group would have received both shots by the end of January, so the researchers compared positive COVID-19 tests between the two age groups after that time. (Before that time, there was little difference between the two groups in terms of positive COVID-19 tests.) They found that that the relative number of positive tests among the fully vaccinated older people was significantly lower than that among the younger group, and hypothesized that the vaccine was in part reducing spread of the virus. They then created models to estimate that the vaccine was lowering the amount of virus by anywhere from 1.5 to 20 times among people who got the shot and then happened to get infected. Still, they measured the amount of virus in people, or viral load, and not the actual rate of transmission from one person to another.

All of which boils down to the fact that it’s not entirely clear yet whether getting vaccinated can stop you from spreading the virus if you’re infected. It might, but the definitive studies to confirm that haven’t been completed yet. So stay tuned. In the mean time, even if you have gotten both shots, follow the advice of public health officials: wear a masks, practice social distancing, and avoid crowded areas, especially indoors. It’s not easy to do, but if more people practice these behaviors, the harder it will be for the virus to continue spreading. And that’s something that we can all agree on.