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The authors present a nonparametric method for estimating vaccine efficacy as a smooth function of time from vaccine trials. Use of the method requires a minimum of assumptions. Estimation is based on the smoothed case hazard rate ratio comparing the vaccinated with the unvaccinated. The estimation procedure allows investigators to assess time-varying changes in vaccine-induced protection, such as those produced by waning and boosting. The authors use the method to reanalyze data from a vaccine trial of two cholera vaccines in rural Bangladesh. This analysis reveals the differential protection and waning effects for the vaccines as a function of blotype and age. Am J Epidemiol 1998; 147:948-59. cholera; communicable diseases; epidemiologic methods; statistics; survival analysis; vaccines Vaccine efficacy (VE) is generally estimated by VE = 1 — RR, where RR is some measure of relative risk in the vaccinated compared with the unvaccinated group. The VE may wane with time, so that the rela-tive risk estimates also change with time. Thus, we would like to estimate the VE at time, t, after vacci-nation, i.e., VE(0 = 1- RR(0- For time-to-even