Diphtheria vaccine is a vaccine used against Corynebacterium diphtheriae, the agent that causes diphtheria. It use has resulted in a more than 90% decrease in number of cases globally between 1980 and 2000. Three initial doses are recommended after which it is about 95% effective. It is effective for about 10 years at which time a booster dose is needed. Immunization may start at six weeks of age with further doses given every four weeks.