We assessed whether Google Flu Trends or CDC ILI Surveillance had a higher correlation with diagnostic tests positive With the exception of the 2003-04 influenza season, correlation coefficients decreased proportionately when seasonal Google Flu Trends or CDC ILI Surveillance were assessed against seasonal CDC Virus Surveillance for the subsequent one or two weeks (Table S1 ). The correlation over the 2003-04 influenza season between Google Flu Trends and CDC Virus Surveillance increased from 0.67 to 0.77 when assessed with a one-week lag and 0.82 when assessed with a two-week lag. CDC ILI Surveillance had a stronger increase from 0.80 to 0.89 with a one-week lag and 0.92 with a two-week lag.