Understanding and Measuring the Dynamics of Infectious Disease Transmission

The challenges of vaccine development-the translation of a basic vaccine concept to a real vaccine product-are numerous and complex. Following successful development, important questions remain as to how, where, and when the vaccine should be used to protect the population. These questions can be answered by understanding the epidemiologic characteristics of disease transmission and using mathematical models to predict the impact of the vaccine and vaccination programs on the population. This chapter begins with an overview of some basic concepts of infectious disease transmission, including the epidemiologic triad of disease causation, modes of infectious disease transmission, and the natural history of infectious diseases. Understanding these concepts is essential to measure, describe, and control infectious diseases. Next, measures of disease frequency used by epidemiologists to assess infectious disease transmission, transmission probability, secondary attack rates, and the basic reproductive number are described. The principle of herd immunity, which is the resistance of a population to invasion and spread of an infectious agent, is covered. Finally, infectious disease modeling, a field growing in importance and increasingly being used to make predictions that inform public health policy, is introduced. The focus is on learning about the basic principles using the susceptible, infectious, and recovered (SIR) model and key parameters rather than on the underlying mathematics, and on appreciating the kinds of questions that might be addressed by applying models to disease control. The foundation of infectious disease epidemiology is based on the tenet that disease occurs not randomly but in patterns reflecting the operation of its underlying causes. Understanding patterns of disease is crucial to preventing disease. However, this is not a simple pursuit. Modeling can help elucidate patterns of infectious disease, and the results can provide a sound scientific base for preventive measures, such as vaccinations.