5 Influences on vaccine efficacy
The efficacy of a protective vaccine is rarely 100 per cent. Vaccine efficacy can be calculated from the secondary attack rates in vaccinated and unvaccinated individuals and expressed as the relative reduction in the risk of infection in vaccine recipients, compared to the risk in unvaccinated people. In this section, we discuss factors that influence vaccine efficacy and determine whether it reaches the level required to protect a recipient against subsequent infection. We are not concerned here with population effects such as herd immunity and critical immunisation thresholds, which influence the ability of a vaccination programme to eliminate an infection in a community (we consider this in Section 7).