ABSTRACT
This chapter discusses aspects of the population transmission dynamics of communicable infectious diseases that are relevant to vaccination programmes. Its focus is primarily conceptual rather than technical, though some key quantitative relationships that apply in important special cases will be presented. The chapter begins by discussing the concepts of infectious contacts, transmission and herd immunity. It then introduces the idea of a reproduction number, and how this relates to the critical immunisation threshold. Next, the chapter describes how these quantities may be estimated in a simple situation known as homogeneous mixing. The chapter then goes on to discuss the complexities that arise in more realistic scenarios with heterogeneous mixing, and finally it describe how models may be used to represent such heterogeneities and to evaluate the likely impacts of vaccination programmes.
