Many respiratory transmissible diseases exhibit seasonal epidemics. Annual periodic forcing causes a wide range of oscillatory epidemic behaviour, as illustrated by incidence rates for measles and pertussis [3] [4] [5] . The source of seasonal forcing in those cases is largely attributed to annual variations in the intensity of contact between children, but a range of other mechanisms have been proposed. In the case of influenza, two recent hypotheses have centred on variation in vitamin D levels and air humidity [6, 7] .