In formulating our inter-city transmission model, we take the city as our unit of study. Each of N cities, i, has an infection time t i , an invariant population size P i and a time-varying mortality rate, r i,t at time t. Infected city i is separated from susceptible city j by distance d ij . Each week, each city can be in one of the three disease states: Susceptible, Latent or Infectious. We assume that all cities are susceptible at the start of a wave, they are latently infected for one week on infection, and that a city becomes infectious the week after it becomes infected. We assume that all transmission is endogenous to England and Wales or US after external seeding to the first infected city in each territory. If a city becomes infected in week t i , the candidate infectors are only those cities that are infectious in week t i . We assume that the transmission parameters are constant through time.