Evaluating Drivers of Influenza Seasonality

To assess the role of putative drivers of influenza seasonality, we collected province-level demographic, economic, and geographic data, including population size and density [20] , gross domestic product [20] , and human mobility patterns between provinces (including the per capita number of passengers travelling by air, rail, road, and boat [21] (link)). Average latitude and longitude coordinates for each province were obtained by weighting the coordinates of cities participating in influenza surveillance by their population sizes (Table 1).
We obtained daily meteorological data for each participating city during the study period, including temperature (minimum, maximum, mean), vapor pressure (minimum, maximum, mean), relative humidity (minimum, maximum, mean), rainfall, and hours of sunshine, as recorded by China Meteorological Administration (Table 1; Text S1) [22] . Province-level meteorological indicators were calculated as population-weighted averages of city-level data. Summary climate indicators were obtained by averaging the daily values of each climate factor by season (winter, Dec–Feb; spring, Mar–May; summer, Jun–Aug; fall, Sep–Nov), as well as calculating annual minimums and maximums. We also categorized the 30 provinces into six climatic zones on the basis of previous work [23] , ranging from tropical to cold-temperate climates (Figure 1; Table 1).