Breakthrough infections might not be a big transmission risk. Here’s the evidence


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Gloria Clemons administers a COVID-19 vaccine to Navy veteran Perry Johnson at the Edward Hines Jr. VA Hospital in Hines, Ill., in September.

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It’s hard to prove that an infected vaccinated person actually was responsible for transmitting their infection to someone else.

«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 says.

There’s new laboratory evidence supporting Kedl’s supposition. 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 only generate the kinds of antibodies that circulate throughout the body.

But that might not be the whole story.

«I think what was the big surprise here is that the mRNA vaccines are going beyond that,» says Michal Caspi Tal, until recently an instructor at the Stanford Institute for Stem Cell Biology and Regenerative Medicine and now a visiting scientist at MIT.

What Tal has found is that in addition to the circulating antibodies, there was a surprisingly large amount of antibodies in mucosal membranes in the nose and mouth, two of the primary entry points for the coronavirus.

The vaccinated aren’t sitting ducks

Immunologist Jennifer Gommerman of the University of Toronto found this as well.

«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,» Gommerman says.

If there are antibodies in the mucosal membranes, they would likely be coating any virus that gets into the nose or throat. So any virus that was exhaled by a sneeze or a cough would likely be less infectious.

Gommerman says until now, it seemed likely that a vaccine that was delivered directly to the mucosal tissue was the only way to generate antibodies in the nose or throat.

«Obviously a mucosal vaccination would be great, too. But at least we’re not sitting ducks,» Gommerman says. «Otherwise everyone would be getting breakthrough infection.»


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Now, these studies by Gommerman and Tal have yet to undergo peer review, and some have already suggested the antibodies they described may not confer 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.

«If you actually isolate virus from people who are getting a secondary infection after being vaccinated, that virus is less good at infecting cells,» Pepper says. «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.»

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, University of Colorado’s Kedl says it’s one more reason getting vaccinated is a good idea.

«Because you’re going to be even more protected yourself. And you’re going to be better off protecting other people.»

Kedl says that’s what you call a win-win situation.

  • breakthrough infections
  • COVID-19 vaccine
  • COVID-19
  • Infectious Disease

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