Close-up of a man in a surgical mask receiving a vaccine against COVID-19.

Vaccinated people are less likely to pass on Omicron than are those who have not been immunized.Credit: Frederic J. Brown/AFP/Getty

People who become infected with the Omicron variant are less likely to spread the virus to others if they have been vaccinated or have had a previous SARS-CoV-2 infection, according to a study in US prisons1. And people who have both had a previous infection and been vaccinated are even less likely to pass on the virus, although the benefit of vaccines in reducing infectiousness seems to wane over time.

The findings are good news, says Megan Steain, a virologist at the University of Sydney, Australia. They show that the more exposure people have to the virus, whether through vaccines, boosters or infections, the “higher the wall of immunity”, she says. “If we can keep high levels of booster vaccinations up, then we can decrease how infectious people are when they’re sick,” says Steain.

The study was posted as a preprint on medRxiv this month and has not been peer reviewed.

Omicron wave

Nathan Lo, an infectious-disease researcher at the University of California, San Francisco, and colleagues analysed data on more than 22,000 confirmed cases of SARS-CoV-2 infection across California’s 35 adult prisons over a 5-month period starting at the end of 2021, when the first wave of Omicron began ripping through the United States. The wave started with the BA.1 subvariant, but by the end of April, BA.2 had overtaken it and was the most common cause of COVID-19 in the country. Previous studies have suggested that vaccinated people are less likely to spread the virus if they were subsequently infected with Delta, but Lo’s study is one of the first to consider whether vaccines and previous infection reduce infectiousness with Omicron.

The team found that among individuals with COVID-19, those who received at least one vaccine shot were 24% less likely to infect close contacts— in this case cellmates — compared with unvaccinated prisoners. People who had been infected before were 21% less likely to infect others compared with prisoners with no previous infection, and those who had been both vaccinated and previously infected were 41% less likely to pass on the virus compared with unvaccinated individuals without a previous infection.

Lo says he is surprised at the dose–response relationships; each vaccine dose a person had reduced the risk of passing on the virus by a further 12%, on average. How recently people had been vaccinated was also important. For every 5 weeks that passed since a person’s last vaccine dose, the risk of transmitting the infection to a close contact increased by 6%.

New infections

Although vaccination and previous infection help to reduce Omicron’s infectiousness, neither was enough to halt the number of new infections among prisoners. Four times out of five, the people who spread on the virus to others had been vaccinated or previously infected.

Prisoners were isolated after they tested positive, but close contacts had usually been exposed for around two days before the COVID-positive individual was isolated, says Lo. Unvaccinated people had a 36% chance of transmitting the virus to close contacts but prisoners who were vaccinated and had had a previous infection still had a 20% risk of spreading the infection.

Next variant

Steain says the findings accord with what researchers know about the virus so far. But as new variants evolve, it is possible that the way they cause infections could change. “If some variants are able to replicate faster, for example, then they might be more infectious because you’re getting higher viral loads, faster, in the nasal cavity, and therefore, you would think, that would equate to more shedding,” says Steain.

Previous studies on transmissibility have often focused on households, where transmission is less likely to occur than in cramped prison cells. And almost all of those studies were done before Omicron, often in people who either weren’t vaccinated or had had only the first set of vaccines.

Allen Cheng, an infectious-disease physician and epidemiologist at Monash University in Melbourne, Australia, is eager to see whether the findings from the prison study are replicated in household studies, which might better reflect how the virus spreads in the community. If replicated, the findings will probably apply to other emerging variants, too, given that most are subvariants of Omicron, he says.

And, although the benefits of vaccination in reducing transmission fade over time, he welcomes the news that previous infection could reduce transmission in a future infection. “Particularly with Omicron, where we’re having so much trouble trying to control it in general, we’re happy with anything we can get,” says Cheng.