ESTIMATING VACCINE EFFICACY IN PROSPECTIVE STUDIES

Vaccine efficacy may be estimated by comparing affected proportions or incidence rates between groups of vaccinates and non-vaccinates. This paper discusses vaccine efficacy assessment and reviews interval esti mation methods for summary measures from clinical trials and cohort st udies of a protective exposure. The measures considered are the attrib utable fraction as the complement of the risk ratio or incidence densi ty ratio, and the difference in risks or incidence densities. A simple formula is developed for the score method of estimating the incidence density ratio. The application of these methods is illustrated using recent reports of veterinary vaccine studies.