Figure 1. Definitions of direct, indirect and total efficacy measures.

Direct efficacy can be estimated through comparison of the attack rate in vaccinated animals (${\displaystyle ARV}$) and unvaccinated animals (${\displaystyle ARU}$) that may be held in the same herd to control for the background infectious pressure or through comparison of fully vaccinated and unvaccinated herds. Indirect efficacy compares the attack rate in unvaccinated animals (${\displaystyle AR{U}_V}$) in a partially vaccinated herd to that in unvaccinated control herds (${\displaystyle ARU}$). Total efficacy compares the attack rate of all animals in a partially vaccinated herd (${\displaystyle AR}$) and unvaccinated control herds (${\displaystyle ARU}$).