The geographical spread of influenza

How infectious diseases spread in space within one cycle of an epidemic is an important question that has received considerable theoretical attention. There are, however, few empirical studies to support theoretical approache s, because data are scarce. Weekly reports obtained since 1984 from a netwo rk of general practitioners spanning the entire French territory allows the analysis of the spatio-temporal dynamics of influenza over a fine spatial scale. This analysis indicates that diffusion over long distances, possibly due to global transportation systems, is so quick that homogeneous global mixing occurs before the epidemic builds up within infected patches. A simp le model in which the total number of cases is given by the empirical time- series and cases are randomly assigned to patches according to the populati on weight of the patches exhibits the same spatio-temporal properties as re al epidemic cycles: homogeneous mixing models constitute appropriate descri ptions, except in the vicinity of the epidemic's peak, where geographic het erogeneities play a role.