Even among vaccines that are effective enough to halt a pathogen on a population level, it doesn't mean it guarantees immunity on an individual level. Some peo

Even among vaccines that are effective enough to halt a pathogen on a population level, it doesn't mean it guarantees immunity on an individual level. Some people vaccinated for smallpox or polio or measles caught them anyway back when they were endemic, but usually with milder disease due to the prior vaccination.