Interpreting Vaccine Efficacy

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Abstract
Phase 111 trials of a new vaccine are usually designed to give the vaccine the greatest chance of showing that it works. Such trials frequently exclude any groups of subjects likely to have a suboptimal immune response and use a case definition for the primary trial end point that is as specific as possible. Vaccine efficacy (VE) that is measured in this way provides important information about the ability of a vaccine to prevent the infection in question, but it does not necessarily provide the information that will be of most value in determining whether the vaccine could make an important contribution to public health. One way that the results of a vaccine trial can be used to illustrate the potential public health impact of the vaccine is through calculation of the number of cases of a variety of clinical syndromes associated with the pathogen in question that might be prevented if the vaccine were introduced. The difference in incidence rates between vaccinated and control subjects is used to calculate a vaccine-attributable reduction (VAR). VE—usually calculated as 1 — (the incidence rate in vaccinated subjects/the incidence rate in the control group)—indicates a proportional reduction in incidence, whereas the VAR measures the absolute reduction. An example of how application of the VAR approach can provide useful information is given in the study by Mahdi et al. [1], published in this issue of Clinical Infectious Diseases.