Vaccine is an immuno-biological substance designed to produce specific protection against a given disease It stimulates the production of protective antibody and other immune mechanisms Vaccines can be prepared from live modified organisms, inactivated or killed organisms, extracted cellular fractions, toxoids or combination of these Vaccine efficacy is the percentage reduction of disease in a vaccinated group of people compared to an unvaccinated group, using the most favorable conditions Vaccine efficacy was designed and calculated by Greenwood and Yule in 1915 for cholera and typhoid vaccines It is best measured using double-blind, randomized, clinical controlled trials, such that it is studied under “best case scenarios