A method is described for estimating the relative incidence of clinical events in defined time intervals after vaccination compared to a control period using only data on cases. The method is derived from a Poisson cohort model by conditioning on the occurrence of an event and on vaccination histories. Methods of analysis for event-dependent vaccination histories and survival data are discussed. Asymptotic arguments suggest that the method retains high efficiency relative to the full cohort analysis under conditions which commonly apply to studies of vaccine safety.