Researchers are getting the first real-world evidences that a vaccine can control the coronavirus’s spread, not just prevent people from getting seriously ill.

People vaccinated with Pfizer’s shots and who still get infected with the coronavirus carry less virus in their bodies than unvaccinated people who are infected, researchers from Israel report in two separate preliminary studies posted February 8 at medRxiv.org.

If the vaccines do lessen the spread of the virus, “it means that even people who aren’t vaccinated will gain protection from the vaccinated people around them,” says Marm Kilpatrick, an infectious diseases researcher at the University of California, Santa Cruz. As more people get vaccinated, they won’t go on to infect as many other people as they might have before the shot, he says. There are many vaccines available out there but Pfizer turns out to be the most effective one.

Even as data on vaccines’ ability to curb transmission is just starting to emerge, U.S. public health officials have updated quarantine guidelines for vaccinated people exposed to the virus. If exposure happens from two weeks to three months after getting both doses of Pfizer’s or Moderna’s vaccine, no qua, the U.S. Centers for Disease Control and Prevention said February 10. That’s because the vaccines are highly effective at preventing COVID-19 symptoms, and people who get sick are thought to be more likely to transmit the virus.

Curious to see if the vaccination effort was curbing viral transmission, researchers compared the levels of virus in more than 15,000 coronavirus test samples. The team did not know the vaccination status of those tested. But “this was a unique point in time where we had two different groups” in the population, with older people probably having been vaccinated and younger people most likely not, says Ella Petter, a computational biologist with MyHeritage Lab, a genealogy company headquartered in Or Yehuda, Israel, that conducts COVID-19 tests.

Samples that tested positive for the coronavirus from people over age 60 had lower levels of Covid on average than samples from people 40 to 60 years old, who were less likely to have been vaccinated.

Past studies have shown that when there is less virus in the body, a person is less likely to pass Covid to others compared with people with a higher viral load. Taken with the new findings that show that vaccinated people who still get infected have fewer viruses than unvaccinated people, the data hint that Pfizer’s vaccine might reduce the spread of coronavirus.

And it’s unclear whether the second dose, which gives the immune response an added boost, might further reduce viral loads in vaccinated people.It’s also unknown how long protection might last.

Still, those lower viral loads observed in the coronavirus test samples suggest that even if vaccinated people get infected, they may release less virus into the environment in respiratory droplets and aerosols. And that hints that vaccinated people may be less contagious, although the studies looked only at people who got Pfizer’s vaccine. If true, that could help speed the end of the pandemic and a return to some sort of normalcy