Improving Google Flu Trends estimates for the United States through transformation

Abstract

Google Flu Trends (GFT) uses Internet search queries in an effort to provide early warning of increases in influenza-like illness (ILI). In the United States, GFT estimates the percentage of physician visits related to ILI (%ILINet) reported by the Centers for Disease Control and Prevention (CDC). However, during the 2012-13 influenza season, GFT overestimated %ILINet by an appreciable amount and estimated the peak in incidence three weeks late. Using data from 2010-14, we investigated the relationship between GFT estimates (%GFT) and %ILINet. Based on the relationship between the relative change in %GFT and the relative change in %ILINet, we transformed %GFT estimates to better correspond with %ILINet values. In 2010-13, our transformed %GFT estimates were within ± 10% of %ILINet values for 17 of the 29 weeks that %ILINet was above the seasonal baseline value determined by the CDC; in contrast, the original %GFT estimates were within ± 10% of %ILINet values for only two of these 29 weeks. Relative to the %ILINet peak in 2012-13, the peak in our transformed %GFT estimates was 2% lower and one week later, whereas the peak in the original %GFT estimates was 74% higher and three weeks later. The same transformation improved %GFT estimates using the recalibrated 2013 GFT model in early 2013-14. Our transformed %GFT estimates can be calculated approximately one week before %ILINet values are reported by the CDC and the transformation equation was stable over the time period investigated (2010-13). We anticipate our results will facilitate future use of GFT.

Figure 1. Weekly percentage of sentinel physician visits related to influenza-like illness (ILI) reported by the Centers for Disease Control and Prevention (CDC) and estimated using Google Flu Trends (GFT), United States, October 2010–March 2014.

The final CDC value (f%ILINet; blue) is compared to the GFT estimate (%GFT; red) and the transformed GFT estimate using c = 0.65 (transformed %GFT; turquoise). The GFT model was recalibrated during the 2013–14 season: dashed lines show the period in which GFT estimates were retrospectively re-estimated using the 2013 GFT model.