Vaccine efficacy is defined as the reduction in the incidence of a disease among people who have received a vaccine

Vaccine

A vaccine is a biological preparation that improves immunity to a particular disease. A vaccine typically contains an agent that resembles a disease-causing microorganism, and is often made from weakened or killed forms of the microbe or its toxins...

 compared to the incidence in unvaccinated people. The efficacy of a new vaccine is measured in phase II or phase III clinical trial

Clinical trial

Clinical trials are a set of procedures in medical research and drug development that are conducted to allow safety and efficacy data to be collected for health interventions...

s 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.
. 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.