As countries around the world (Canada/U.S, Europe/Asia Africa/Aus S.America) roll out vaccines that prevent COVID-19, studies are underway to determine whether shots can also stop people from getting infected and passing on the SARS-CoV-2 virus. Vaccines that prevent transmission could help to bring the pandemic under control if they are given to enough people.

What The Research Says

Preliminary analyses suggest that at least some vaccines are likely to have a transmission-blocking effect. But confirming that effect — and how strong it will be — is tricky because a drop in infections in a given region might be explained by other factors, such as lockdowns and behavior changes. Not only that, the virus can spread from asymptomatic carriers, which makes it hard to detect those infections.

Although most clinical trials of COVID-19 vaccines showed that vaccines prevented the disease, some trial results also offered clues that shots might prevent infection. A vaccine that is highly effective at preventing people from acquiring the infection in the first place would help to reduce transmission

During the trial of Moderna's vaccine, researchers swabbed all participants to see if they had any viral RNA. They saw a two-thirds drop in the number of asymptomatic infections among people who received the first shot of the two-dose vaccine, compared with those who received a placebo. But they tested people only twice, about a month apart, so might have missed infections.

Pfizer, another leading COVID-19 vaccine, says that it will start swabbing participants every two weeks in vaccine trials taking place in the United States and Argentina, to see whether the shot can prevent infection.

Can Vaccines Make People Less infectious?

There is no hard evidence that vaccines won’t stop or significantly lessen the chances of infection. But jabs might make infected people less able to pass the virus on, or make them less infectious, and so reduce transmission.

Several research groups in Israel are measuring ‘viral load’ — the concentration of viral particles in vaccinated people who later test positive for SARS-CoV-2. Researchers have found that viral load is a good proxy for infectiousness.

One team observed a significant drop in viral load in a small number of people infected with SARS-CoV-2 in the two to four weeks after receiving their first dose of the Pfizer vaccine, compared with those who caught the virus in the first two weeks after the injection.

 “The data is certainly intriguing and suggestive that vaccination may reduce the infectiousness of COVID-19 cases, even if it does not prevent infection altogether.” 

 - Virginia Pitzer -Infectious Diseases Yale  

The Oxford–AstraZeneca trial also observed a larger reduction in viral load in a small group of vaccinated participants than in the unvaccinated group. Whether these observed reductions in viral load are sufficient to make someone less infectious in real life is not yet clear, say, researchers.

Will Vaccination Stop You From Spreading The Virus?

If vaccines created what’s known as sterilizing immunity all the time, no vaccinated person would transmit the virus. Vaccinated grandparents could safely play with their unimmunized grandchildren. Countries could let people travel safely and welcome visitors who had proof of vaccination with little fear of introducing new viral variants or reigniting outbreaks.

That level of assurance is a tall order. Few vaccines, for any infectious diseases, create sterilizing immunity—even the most effective ones. The inactivated poliovirus vaccine developed by Jonas Salk did little to block infection or transmission of the virus, yet it powerfully prevented paralytic polio. By 1961, 6 years after it was licensed, only 54% of the U.S. population had received the vaccine, yet paralytic polio cases had dropped by more than 90%.