Vaccinations are used to initiate an immune response using a form of the pathogen/antigen that will not result in infection. From the graph, the more booster shots, the longer the period of time the person is considered immune. Cases occurring in people aged over 65 would be the result of not having a booster for a long time. The reduction in numbers of tetanus cases would suggest that vaccinations are effective while the recurring schedule ensures a person maintains immunity.