Booster immunization can lead to rapid induction of protective immunity against pathogens (e.g. ≦7 days after the booster dose). This rapid response means that booster immunization can be used at short notice prior to an event that might require an activated immune response. For instance, a subject can be primed at a young age against a pathogen that typically affects more elderly subjects under specific circumstances such that, when the subject is older, the immune response can be mobilized rapidly if those specific circumstances are expected. An example would be to prime a subject against infections that are typically acquired nosocomially and then, soon before a scheduled hospital appointment, to boost their immune response so that they enter hospital in an immune-alert state.