RESUMO
In this paper we demonstrate that immunization session size distributions are governed by binomial statistics and determined by just two readily available programmatic parameters, the birth cohort of the catchment area and the session frequency. Given this new knowledge, the unavoidable component of an immunization facility's opened vial wastage rate can be determined algebraically for each vial size and discard time. This has significant positive programmatic implications for immunization session planning, vaccine needs forecasting, vaccine wastage monitoring, and vaccine product specification and choice. Further operational research is required to field test the different applications of the model and to inform the development of tools and guidance.