The ability to assess the protective efficacy of a vaccine by measuring the proportion of vaccinees who generate a particular immune response, without having to measure clinical outcomes, has significant advantages. The availability and quality of such substitute endpoints are important for vaccine development, licensure and effectiveness monitoring. A better understanding of the interrelationships between vaccination, the immune response, protection, and clinical outcomes is thus of interest not only to regulatory and public health authorities but also to microbiologists, immunologists, epidemiologists and statisticians [WHO/IVB/13.01].
The correlates of vaccine-induced immunity are a subject of continued interest for both theoretical and practical reasons. The latter include the need to evaluate the consistency of vaccine production; the susceptibilities of individuals and populations after vaccination; the validation of vaccines for which placebo-controlled efficacy trials are no more ethical (e.g. Rotavirus vaccines), as when a prior-generation vaccine is already licensed; and the licensure of combination vaccines.