ABSTRACT
This chapter focuses on concepts of effectiveness, whereas Chapters 13 to 16 describe the methods for estimating vaccine effectiveness in the field and the methodological issues involved. Since most field studies focus on protective effectiveness (the degree of protection that the vaccine imparts directly to an individual, by reducing that individual’s risk of infection or disease) this is where we begin in the first section of the chapter. In line with the book’s focus on public health epidemiology, we concentrate on the effectiveness of licensed vaccines used in the field, rather than on the process of demonstrating a vaccine’s efficacy for scientific or licensure purposes. The distinction between field effectiveness and experimental efficacy is discussed in the second section. In the third section, we discuss different concepts of protective vaccine effectiveness according to which outcome is studied: disease, infection or progression of disease (severity). Since from a disease control perspective the effectiveness of vaccination in reducing infectiousness is of key importance, we have dedicated the fourth section of the chapter to this. This brings us to the chapter’s final section, in which we combine different measures of vaccine effectiveness into a single one, the vaccine effectiveness against transmission, and discuss how this relates to the indirect effects of vaccination and reproduction numbers.
