Breakthrough Infections May Not Be as Big a Threat as Researchers Once Thought

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A person rolls up their sleeve as they sit down on a chair, as a person in a white lab coat works at a table behind them.
A pharmacist prepares to give a Pfizer COVID-19 booster vaccination to a person at a vaccination booster shot clinic on Oct. 1, 2021, in San Rafael, California. (Justin Sullivan/Getty Images)

Conventional wisdom says that if you're vaccinated and you get a breakthrough infection with the coronavirus, you can transmit that infection to someone else and make that person sick.

But new evidence suggests that even though that may happen on occasion, breakthrough infections might not represent the threat to others that scientists originally thought.

Ross Kedl, an immunologist at the University of Colorado School of Medicine, will point out to anyone who cares to listen that basic immunology suggests the virus of a vaccinated person who gets infected will be different from the virus of an infected unvaccinated person.

That's because vaccinated people have already made antibodies to the coronavirus. Even if those antibodies don't prevent infection, they still "should be coating that virus with antibody and therefore helping prevent excessive downstream transmission," Kedl says. And a virus coated with antibodies won't be as infectious as a virus not coated in antibodies.

A woman sits at a table in the corner of a high-ceilinged room, beyond some chairs for waiting.
A pharmacist fills a syringe with the Pfizer COVID-19 booster vaccination at a vaccination booster shot clinic on Oct. 1, 2021, in San Rafael. (Justin Sullivan/Getty Images)

Not enough evidence

In Provincetown, Mass., this summer, a lot of vaccinated people got infected with the coronavirus, leading many to assume that this was an example of vaccinated people with breakthrough infections giving their infection to other vaccinated people.

Kedl isn't convinced.

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

In a recent Israeli study of breakthrough infections among health care workers, the researchers report that in "all 37 case patients for whom data were available regarding the source of infection, the suspected source was an unvaccinated person."

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 generate only 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 Stanford University's Institute for Stem Cell Biology and Regenerative Medicine and now a visiting scientist at the Massachusetts Institute of Technology.

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.

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"We're not 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 got into the nose or throat. So any virus that was exhaled by a sneeze or a cough would likely be less infectious.

Gommerman says that 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."

These studies by Gommerman and Tal have yet to undergo peer review, and some already have suggested that the antibodies they have 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, an immunologist at the University of Washington, 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.

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"It's not known why. Is it covered with an antibody? Maybe," she said. "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, the University of Colorado's Kedl says it's one more reason that 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.

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