Vaccines represent our most powerful tool for disease prevention. However, no medical treatment can guarantee 100% effectiveness, and the success rate of a vaccine can vary based on its formulation and the specific disease it targets. Differences in individual immune responses mean that the vaccine’s protective effect may differ from person to person, and in rare instances, full immunity might not develop. Still, vaccines are highly effective for most individuals. When a large enough portion of the population is vaccinated, those with incomplete immunity benefit from herd immunity. Even if a vaccinated child gets sick, their symptoms are typically much less severe than those in unvaccinated children.