We consider the statistical evaluation and estimation of vaccine efficacy w
hen the protective effect wanes with time. We reanalyse data from a 5-year
trial of two oral cholera vaccines in Matlab, Bangladesh. In this field tri
al, one vaccine appears to confer better initial protection than the other,
but neither appears to offer protection for a period longer than about 3 y
ears. Time-dependent vaccine effects are estimated by obtaining smooth esti
mates of a time-varying relative risk RR(t) using survival analysis. We com
pare two approaches based on the Cox model in terms of their strategies for
detecting time-varying vaccine effects, and their estimation techniques fo
r obtaining a time-dependent RR(t) estimate. These methods allow an explora
tion of time-varying vaccine effects while making minimal parametric assump
tions about the functional form of RR(t) for vaccinated compared with unvac
cinated subjects.