OUTBREAKS IN HIGHLY VACCINATED POPULATIONS - IMPLICATIONS FOR STUDIESOF VACCINE PERFORMANCE

Most of the factors associated with the failure of a Vaccination to pr ovide protective immunity are not distributed uniformly or randomly wi thin populations. This paper explores the extent to which a nonrandom distribution of vaccination failures and the selection of exceptional situations for investigation may influence estimates of vaccine perfor mance. The authors show that outbreak investigations will tend to unde restimate vaccination efficacy, and that the extent of underestimation will be related directly to the size of the epidemic triggering an in vestigation, the vaccination coverage in the community, and the extent of clustering of vaccination failures in the population; it will be r elated inversely to the size of and contact intensity within the inves tigated community. These potential sources of bias are not the only pr oblems that arise in estimating vaccine efficacy, but they should be t aken into consideration when analyzing and interpreting outbreak situa tions. The fact that outbreak investigations carried out within the Un ited States during the past decade have provided estimates of measles vaccination efficacy on the order of 95% is consistent with a somewhat higher overall ''true'' efficacy of current vaccines and procedures i n the total population. It is important to understand better the frequ ency, distribution, and risk factors for vaccination failures in popul ations.