We express this variation in terms of peak R 0 and relative amplitude of variation throughout. The sinusoidal form is a good match for variation in infectiousness as a function of absolute humidity in temperate regions [14] , but we also examine the effect of using school terms as a seasonal driver using step-function forcing (see the electronic supplementary material). In determining feasible ranges for these parameters, we reviewed the range of estimates that exist for R 0 . The majority of these are calculated for the major global epidemics (1918, 1957, 1968) , since the antigenic novelty of pandemic viruses means an assumption of a serologically naive population can be made, making analysis simpler. For seasonal influenza, knowledge of population susceptibility is necessary to estimate R 0 (as opposed to the effective reproduction number, R). Estimated values for the 1918 pandemic range from 1.3 to 2.8 [20] . Similar values are found for the 1957 and 1968 epidemics [21] . These values also correspond well with those from studies of seasonal influenza, giving winter R 0 of 1.7 and school holiday R 0 of 1.4 [16] .