Vaccines provide our body with memory antibody reserves which tend to decline over a period of time. Infections which have the ability to progress very quickly to become a disease, does not give our memory immune response, enough time to prevent a disease. We need to have booster shots to prevent these diseases. Booster shots are extra doses of primary vaccine administered early in life. These shots exposes our immune system to the antigen again and therefore, serves a reminder to it to gear up again in its full strength for specific fight against the future exposure to the disease causing pathogen. Killed or inactivated vaccines provide immunity for a shorter duration and thus, require booster shots.