The use of regional surveillance comparisons did not result in increased correlation with virologic surveillance of the subsequent one or two weeks. The mean Google Flu Trends correlation with CDC Virus Surveillance was 0.70 (SD 0.05) at baseline, 0.70 (SD 0.07) with a one-week lag, and 0.63 (SD 0.11) with a two-week lag ( Table S2 ). The mean CDC ILI Surveillance correlation with CDC Virus Surveillance decreased from 0.78 (SD 0.04) at baseline to 0.76 (SD 0.08) with a one-week lag and 0.68 (SD 0.11) with a two-week lag. Calendar week of peak influenza activity per influenza surveillance year for each of the three surveillance systems can be found in Table S3 .