We compiled a US city dataset (figure 1b) from five publications reporting the Weekly Health Index as collated by the Bureau of the Census [24, [30] [31] [32] [33] . It covers the period 14 September 1918 to 15 March 1919 and contains weekly pneumonia and influenza death counts for 47 cities in the US. The US data therefore covers a period of two waves, with not all cities experiencing the later wave. There is very good agreement between different sources where they overlap. We used the Euclidean distance between cities (accounting for curvature of the Earth) to measure separation. Further information is given in the electronic supplementary material.