Evidence mounts that people with breakthrough infections can spread Delta easily

A new study finds that this dominant variant can grow in the noses of vaccinated people as strongly as in unvaccinated people.

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A healthcare worker administers a dose of the Pfizer-BioNTech Covid-19 vaccine at a vaccination site in Lake Worth, Florida, on Friday, Aug. 13, 2021.

A preliminary study has shown that in the case of a breakthrough infection, the Delta variant is able to grow in the noses of vaccinated people to the same degree as if they were not vaccinated at all. The virus that grows is just as infectious as that in unvaccinated people, meaning vaccinated people can transmit the virus and infect others.

Previous studies in hospitals in India; Provincetown, Massachusetts; and Finland have also shown that after vaccine breakthrough infections with Delta, there can be high levels of virus in people’s nose whether they are vaccinated or not. The next logical step was to determine whether vaccinated people could shed infectious virus. Many experts suspected they did, but until this study it hadn’t been proven in the lab.

“We're the first to demonstrate, as far as I'm aware, that infectious virus can be cultured from the fully vaccinated infections,” says Kasen Riemersma, a virologist at University of Wisconsin who is one of the authors of the study.

“Delta is breaking through more preferentially after vaccines as compared to the non-Delta variants” because it’s extremely infectious and evades the immune response, says Ravindra Gupta, a microbiologist at University of Cambridge. Gupta’s lab was one of the first to document that fully vaccinated healthcare workers could get infected with Delta and had high levels of virus in their noses.

If the Wisconsin study finding holds up, then people with breakthrough infections—many of whom do not develop COVID symptoms—can unknowingly spread the virus. “It...

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