Correlations among influenza surveillance systems differed by influenza season. The correlation coefficient describing the association between Google Flu Trends and CDC Virus surveillance ranged from 0.67 (95% CI 0.43, 0.82) during the 2003-04 influenza season to 0.94 (95% CI 0.89, 0.97) during the 2004-05 influenza season ( Table 2 ). The mean correlation coefficient for these comparisons was 0.79 (SD 0.13). The correlation between CDC ILI Surveillance and CDC Virus Surveillance ranged from 0.79 (95% CI 0.62, 0.89) during the 2005-06 influenza season to 0.94 (95% CI 0.88, 0.97) during the 2004-05 influenza season. The mean correlation coefficient for these comparisons was 0.86 (SD 0.07).