Woolthuis RG, et al. - Researchers here develop a range of mathematical transmission models to investigate the effect of variable duration of immunity on the size of seasonal epidemics. As per the models, variation in the duration of immunity impacts the long-term effectiveness of vaccination, and that vaccine effectiveness cannot be judged for each year in isolation. These findings seem to have utility for designing vaccination strategies aiming to maximize the vaccination coverage while extending the age range of persons eligible for vaccination.