 It is best measured using double-blind, randomized, clinical controlled trials, such that it is studied under “best case scenarios.”  Vaccine effectiveness differs from vaccine efficacy in that vaccine effectiveness shows how well a vaccine works when they are always used and in a bigger population whereas vaccine efficacy shows how well a vaccine works in certain, often controlled, conditions.Vaccine efficacy studies are used to measure several possible outcomes such as disease attack rates, hospitalizations, medical visits, and costs.The outcome data (vaccine efficacy) generally are expressed as a proportionate reduction in disease attack rate (AR) between the unvaccinated (ARU) and vaccinated (ARV), or can be calculated from the relative risk (RR) of disease among the vaccinated group.