evidence: Some fully vaccinated people—and those who have received boosters—have acquired SARS-CoV-2 and developed COVID-19. That is expected. One Mayo Clinic intensivist explains what to know about COVID-19 vaccination, boosters and breakthrough infections. [...] The agency defines a breakthrough COVID-19 infection as "a small percentage of fully vaccinated persons" who "will still get COVID-19 if they are exposed to the virus that causes it." To that end, these vaccine breakthrough cases mean that "while people who have been vaccinated are much less likely to get sick, it will still happen in some cases." [...] One reason why fully vaccinated people might develop breakthrough COVID-19 infections "is the characteristics of the vaccine itself and how efficacious the vaccine is, because there is not a single vaccine that we know of that is 100% effective," explained Dr. Sanghavi. "We know from initial trials from both mRNA vaccines that the effectiveness was somewhere around 94% and 95%—slightly lower in the Johnson & Johnson vaccine."