Why Some People Haven’t Caught COVID despite Being Exposed
When scientists exposed people to the virus that causes COVID, only a subset got sick. Studying them could offer clues to immunity
Not everyone who is exposed to SARS-CoV-2, the virus that causes COVID, gets the disease. New research published in June in Nature may partly explain why: In a 2021 experiment called a challenge study, researchers attempted to infect 16 unvaccinated volunteers with a low dose of the original strain of SARS-CoV-2. Participants who quickly squelched the virus activated a fast-acting innate immune response in their nose.
In the study, the team first assessed the participants’ baseline immune function and then spritzed an infectious solution into their nose. For four weeks afterward, the researchers collected nasal swabs and blood samples from the subjects for single-cell DNA sequencing analysis. This offered the scientists “an unprecedented view of what happens at the very early stage of viral exposure in humans,” says Akiko Iwasaki, an immunologist at the Yale School of Medicine, who was not involved in the study.
After inoculation, six of the 16 subjects developed a full-blown COVID infection, accompanied by mild symptoms such as a sore throat and nasal congestion. Three others tested positive or nearly positive in an on-and-off fashion, but had very mild or no symptoms. These individuals’ so-called transient infections were weak and short-lived. And the remaining seven participants resisted infection altogether, never testing positive.
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The three people who fought off transient infections produced heavy doses of signaling proteins called interferons in their…
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