Change in AIC values for the spatial models using geographic distance, work commutes and domestic air traffic as the links between locations driving inter-city spread, compared to the model with the minimum AIC in each season.

The number of cities available for analysis is based on the subset of 310 cities for which onset time could be estimated and which could be successfully matched with county-level commute data from the census. Spatial models were not fit to the 2008/2009 epidemic because of lack of detectable influenza onset times in a sufficient number of locations.