Measures of how well a vaccine works

Abstract

This article discusses definitions of the effect of vaccines across outcomes of interest, from individual level to contacts of vaccinated people to population level. The focus is on the outcome evaluated in an observational study on the effectiveness of mpox vaccine in the context of potential outcomes that could have been measured. When evaluating how well a vaccine works, two categories of outcomes are of interest: infection related outcomes, sometimes referred to as infection blocking effects of vaccines, and disease related outcomes, often referred to as symptomatic disease, morbidity, or mortality outcomes.1 These two categories can then be further divided to reflect those in whom the outcomes are measured: the individual who received the vaccine (direct effects), among contacts of the person who received the vaccine (indirect effects), or at population level (herd immunity).2 Direct effects of a vaccine include reducing the chances of a vaccinated individual becoming infected if exposed to the pathogen (ie, reduced susceptibility) and disease related outcomes despite acquiring infection.1 For some infectious diseases, the direct, protective effect of a vaccine against infection is the primary goal, such as with rabies, whereas for many the added downstream benefit of a …