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 circumstances in the community.
Famous quotes containing the word efficacy:
“If there is a case for mental events and mental states, it must be that the positing of them, like the positing of molecules, has some indirect systematic efficacy in the development of theory.”
—Willard Van Orman Quine (b. 1908)