A dynamic model of the transmission of hepatitis B virus (HBV) was used to investigate some of the possible implications of the loss of vaccine-induced immunity on the effectiveness of mass HBV vaccination in high endemicity countries. The model, which describes the processes of horizontal and vertical transmission of HBV in a rapidly growing population, was based on one previously published, modified to allow for waning vaccine-induced immunity. All model parameters were empirically based. Summaries of published data provided estimates of the mean duration of immunity in vaccine responders and age-dependence in the risk of developing acute HBV disease. Simulations of the temporal and long-term impact of mass infant immunization on the prevalence of HBV chronic carriage and acute disease suggested that booster doses are of little benefit in coping with waning vaccine protection. The key factor is that vaccine-induced protection should be long enough to prevent infection in the first few years of life, when the risk of developing chronic carriage is highest.