Comparison of inter-epidemic period and serotype persistence between model (i) (<i>top</i>) and model (ii) (<i>bottom</i>).

<p>(a) As enhancement increases so too does the epidemic period observed in each model. There is also a trend towards longer periods at lower . However, these trends both appear to be stronger in model (ii). (b) The risk of stochastic extinction within the model is shown as the proportion of time in each model that the prevalence of a particular serotype exists above a specific threshold. In both models there is a low risk of extinction but the risk increases with enhancement; again, this trend is stronger in model (ii).