We replicated the analysis that validated Google Flu Trends by assessing the correlation with regional CDC ILI Surveillance data during the 2007-08 influenza season ( Table 4 ). The mean correlation coefficient by US Census Region during the 2007-08 influenza season was 0.97 (SD 0.02), identical to the previously published result [2] . When regional Google Flu Trends data were compared to CDC Virus Surveillance, the mean correlation coefficient was lower, 0.87 (SD 0.04). Additional seasonal analyses by region demonstrated that the mean Google Flu Trends-CDC Virus Surveillance correlation coefficient was consistently below the mean correlation between CDC ILI Surveillance and the reference standard by US Census Region (Table S4) .