Vaccine efficacy

Vaccine efficacy is defined as the reduction in the incidence of a disease among people who have received a vaccine compared to the incidence in unvaccinated people. It is usually measured in a randomised controlled trial (RCT) The efficacy of a new vaccine is measured in phase II or phase III clinical trials by giving one group of people a vaccine and comparing the incidence of disease in that group to another group of people who do not receive the vaccine. Ideally the trial is a randomized controlled trial (preferably "double-blind").

The basic formula is written as:

VE = (ARU - ARV)/ARU (x 100)

where

VE = vaccine efficacy;

ARU = attack rate in the unvaccinated population

and

ARV = attack rate in the vaccinated population.

Vaccine efficacy differs from vaccine effectiveness in the same way that an explanatory clinical trial differs from an intention to treat trial: vaccine efficacy shows how effective the vaccine could be given ideal circumstances and 100% vaccine uptake; vaccine effectiveness measures how well a vaccine performs when is used in routine cirumstances in the community.