Breakthrough infection
In order for a vaccine to be effective, a person must respond to the pathogens in a vaccine through the adaptive branch of the immune system and that response must be stored  in an individual's immunological memory. It is possible for an individual to neutralize and clear a pathogen through the humoral response without activating the adaptive immune response. Vaccines with weaker or fewer strains of a pathogen may primarily cause the humoral response: As a consequence, they fail to ensure future immunity.