Shaman et al. [14] use a similar SIRS model to investigate the possibility that seasonal changes in absolute humidity can generate recorded patterns in pneumonia and influenza mortality data. The model was stochastic, has no age structure, and effectively uses only one strain. Best-fit parameter values are similar to those found in this work, although the relative amplitude of variation in R 0 is in the range 0.4-0.5, significantly higher than our findings. Best-fit parameter sets showed considerable lack of correlation with each other, which the authors attribute to the stochastic nature of their model. Our work suggests that this scatter may be the result of the complex bifurcational structure of such models. As already discussed, our results indicate that a two strain model without cross-immunity or age structure is unlikely to fit patterns of seasonal influenza from a temperate region. However, there are several significant differences between the two systems. Shaman et al. employ a significantly higher background force of infection than ours (approx. 7.3 Â 10 24 yr -1 ), which would place our model in a strongly annual or biennial regime. Hence that model may reproduce the mean attack rate well, but not its variability.