ABSTRACT
This chapter discusses the study of the protection afforded by vaccination. The concept of vaccine efficacy is introduced, and the difference between direct and indirect effects is explained. The implications on study design of various interpretations of a figure for vaccine efficacy gets a mention. The concept of immunity and the measurement of vaccine-induced immunity are central to infectious disease epidemiology and really have few counterparts in the study of non-infectious diseases. The ideal way to measure the protective effect, or efficacy, of a vaccine is to perform a regular randomized, controlled clinical trial, where one group of subjects is given the real vaccine and another is given placebo. Both methods have been used for investigations of potential adverse reactions to vaccines, but the self-controlled case series more frequently. Since vaccine studies assessing actual incidence tend to be big and lengthy projects, an often used alternative is to measure antibody levels in vaccinated subjects.
