Fig. 1

Variation in the duration of immunity increases the height of epidemic peaks. In the absence of vaccination, a pattern of regular epidemics is observed if the duration of immunity is constant (blue line), while an irregular pattern with alternating small and large epidemics is observed if the duration of immunity is variable (red line). After the introduction of vaccination (vertical gray line), the pattern of regular epidemics persists in the constant immune duration scenario, albeit at a lower level (blue line). If the duration of immunity is variable, the irregular pre-vaccination pattern of alternating small and large epidemics is exacerbated (red line). The mean of the prevalence peaks is substantially higher in the variable immunity scenario than in the constant immune duration scenario (dotted lines). The means of the prevalence peaks are calculated over 1,000 seasons using a burn-in period of 50 years. See Methods for details