AUDIE CORNISH, HOST:
Conventional wisdom says if you're vaccinated and you get a breakthrough infection with the coronavirus, you can transmit that infection to someone else and make them sick. But as NPR's Joe Palca reports, new evidence suggests even though that may happen, breakthrough infections may not represent the threat to others that scientists originally thought.
JOE PALCA, BYLINE: In Provincetown, Mass., this summer, a lot of vaccinated people got infected with the coronavirus. And the assumption was that this was an example of vaccinated people with breakthrough infections giving their disease to other vaccinated people. But Ross Kedl says there's a problem with that conclusion.
ROSS KEDL: In all these cases where you have these big breakthrough infections, there's always unvaccinated people in the room.
PALCA: Kedl is an immunologist at the University of Colorado School of Medicine. He says it's hard to prove that an infected vaccinated person actually was responsible for transmitting their infection to someone else.
KEDL: I have seen no one report actually trying to trace whether or not the people who were vaccinated and got infected are certainly downstream and could only possibly be downstream of another vaccinated person.
PALCA: One of the reasons Kedl thinks people with breakthrough infections are less likely to infect someone else is some new research, albeit preliminary, on what kinds of antibodies the vaccines provoke. Initially, most vaccine experts predicted that mRNA vaccines, like the ones made by Pfizer and Moderna, that are injected into someone's arm muscle would generate the kinds of antibodies that circulate throughout the body. Michal Caspi Tal is a visiting scientist at MIT.
MICHAL CASPI TAL: I think what was the big surprise here is the extent to which the mRNA vaccines are going beyond that.
PALCA: What Tal has found is that in addition to the circulating antibodies, there are a surprisingly large amount of antibodies in mucosal membranes in the nose and mouth, two of the primary entry points for the coronavirus. Jennifer Gommerman of the University of Toronto found this as well.
JENNIFER GOMMERMAN: 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.
PALCA: If there are antibodies in the mucosal membranes, they would be coating any virus that gets into the nose or throat. So any virus that was exhaled by a sneeze or cough would likely be less infectious. Gommerman says most scientists have assumed that a vaccine would have to be delivered directly to the mucosal tissue to generate antibodies in the nose or throat.
GOMMERMAN: Obviously, a mucosal vaccination would be great, too, but at least we're not sitting ducks. Otherwise, everyone would be getting breakthrough infection.
PALCA: Now, these studies by Gommerman and Tal have yet to undergo peer review, and some have already suggested the antibodies may not really be true mucosal immunity. But there's other evidence that a vaccinated person's breakthrough infection may not transmit efficiently to others. Marion Pepper is an immunologist at the University of Washington. She says a recent study from the Netherlands looked at how well virus from vaccinated people could infect cells in the lab. Pepper says the answer was not well.
MARION PEPPER: If you actually isolate virus from people who are getting a secondary infection after being vaccinated, that virus is less good at infecting cells. And so it's not known why. You know, 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.
PALCA: More studies are emerging that suggest there's something different about the virus coming from a vaccinated person, something that may help prevent transmission. Whatever it is, Ross Kedl says it's one more reason getting vaccinated is a good idea.
KEDL: Because you're going to be even more protected yourself and you're going to be better off protecting other people.
PALCA: Kedl says that's what you call a win-win situation. Joe Palca, NPR News.
(SOUNDBITE OF NICK BOX'S "THOUGHTS") Transcript provided by NPR, Copyright NPR.