Although the conventional wisdom is that people with breakthrough infections can still transmit the coronavirus to others, new research suggests vaccination may change the virus and make it less transmissible, Joe Palca reports for NPR's "Shots."

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Limited evidence for coronavirus transmission from breakthrough infections

In July, an internal CDC document analyzed an outbreak of 882 Covid-19 cases in Provincetown, Mass., caused by the delta variant. The analysis found that 74% of infected individuals had been vaccinated, and that vaccinated and unvaccinated people carried similar large viral loads in their noses and throats—suggesting that those with breakthrough infections could transmit the virus to others.

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However, Ross Kedl, an immunologist at the University of Colorado School of Medicine, said there was no definitive evidence that the breakthrough infections in the Provincetown outbreak were transmitted by vaccinated individuals.

"I have seen no one report actually trying to trace whether or not the people who were vaccinated who got infected are downstream—and certainly only could be downstream—of another vaccinated person," Kedl said.

He added, "In all these cases where you have these big breakthrough infections, there's always unvaccinated people in the room."

A study of breakthrough infections among health care workers in Israel may also lend support to Kedl's belief, Palca reports. In the study, the researchers wrote that in "all 37 case patients for whom data were available regarding the source of infection, the suspected source was an unvaccinated person."

Vaccination may change the virus and help prevent transmission, early research suggests

According to Kedl, the virus of a vaccinated individual who is infected will be different from the virus of an unvaccinated person who is infected. Vaccinated people have antibodies against the virus, and the antibodies "should be coating that virus ... and therefore helping prevent excessive downstream transmission" even if they don't prevent infection, Kedl said.

One example supporting the idea of reduced coronavirus transmission from vaccinated individuals is a preprint study from the Netherlands, which found that virus from vaccinated individuals was less effective at infecting cells in a lab compared to virus from unvaccinated individuals, Palca writes.

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"If you actually isolate virus from people who are getting a secondary infection after being vaccinated, that virus is less good at infecting cells," said Marion Pepper, an immunologist at the University of Washington who was not involved in the study. "It's not known why. Is it covered with an antibody? Maybe. Has it been hit by some other kind of immune mediators, cytokines, things like that? Maybe. Nobody really knows. But the virus does seem to be less viable coming from a vaccinated person."

In addition, two new preprint studies suggest that mRNA Covid-19 vaccines also produce large amounts of antibodies in the mucosal membranes of the nose and mouth—two of the main entry points of the coronavirus—and not just circulating antibodies in the blood like vaccine experts initially believed.

Jennifer Gommerman, an immunologist from the University of Toronto and an author of one of the preprint studies, said, "This is the first example where we can show that a local mucosal immune response is made, even though the person got the vaccine in an intramuscular delivery."

If there are antibodies in mucosal membranes, they would likely coat any virus entering the nose or throat, Palca writes, which means that any virus exhaled through a sneeze of cough would be less infectious.

According to Gommerman, while a mucosal vaccine would also be beneficial, the results of the preprint studies suggest vaccinated people are not "sitting ducks" when it comes to Covid-19 antibodies, "[o]therwise everyone would be getting breakthrough infection[s]."

Although the current research is early and has not yet been peer-reviewed, more emerging research suggests there is something different about a virus that has infected a vaccinated individual, which may make it less transmissible, Palca reports.

Whatever it is that might make the virus less transmissible in vaccinated people, Kedl said it is just one more reason why getting vaccinated against Covid-19 is a good idea. It is a "win-win" situation "[b]ecause you're going to be even more protected yourself," he said. "And you're going to be better off protecting other people." (Palca, "Shots," NPR, 10/12)