For England and Wales, there is a relatively good agreement between observed and simulated epidemic curves (figure 6b). The observed epidemic curve rises more steeply than the simulation curves in the early stages of the epidemic, and peaks one week earlier than the simulation mean. This suggests that the model may underestimate the external infection pressure early in the wave. We calculated the probability that a city was infected in each week given the observed behaviour of all other cities up to that time. In England and Wales, 245 of 246 cities lie within the 95 per cent interval of their expected distribution. Figure 6a shows the cities which the observed infection week lies outside the stricter inter quartile interval. For further information see the electronic supplementary material. There are no population size ( p ¼ 0.36) or density trends ( p ¼ 0.11) in these cities, which are typically infected later in the epidemic ( p , 0.01 for difference in infection week). In the US, all cities lie within the 95 per cent probability interval and all but three lie within the inter quartile interval. Those three outlier cities are smaller than other cities ( p ¼ 0.01) but equally distributed in space ( p ¼ 0.88) and time ( p ¼ 0.06).