It's called the "Outbreak Effect"...in situations where the virus is spreading due to lower vax rate
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The outbreak effect: When low vaccination rates cause outbreaks, the absolute number of circulating viruses in the community goes up. Because more virus is present, a vaccinated person's chance of exposure increases slightly compared to when elimination settings keep the virus entirely absent. However, they are still overwhelmingly protected compared to anyone who is unvaccinated.
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Btw, got a link to your source?