Reassessing Google Flu Trends data for detection of seasonal and pandemic influenza: a comparative epidemiological study at three geographic scales

Abstract

The goal of influenza-like illness (ILI) surveillance is to determine the timing, location and magnitude of outbreaks by monitoring the frequency and progression of clinical case incidence. Advances in computational and information technology have allowed for automated collection of higher volumes of electronic data and more timely analyses than previously possible. Novel surveillance systems, including those based on internet search query data like Google Flu Trends (GFT), are being used as surrogates for clinically-based reporting of influenza-like-illness (ILI). We investigated the reliability of GFT during the last decade (2003 to 2013), and compared weekly public health surveillance with search query data to characterize the timing and intensity of seasonal and pandemic influenza at the national (United States), regional (Mid-Atlantic) and local (New York City) levels. We identified substantial flaws in the original and updated GFT models at all three geographic scales, including completely missing the first wave of the 2009 influenza A/H1N1 pandemic, and greatly overestimating the intensity of the A/H3N2 epidemic during the 2012/2013 season. These results were obtained for both the original (2008) and the updated (2009) GFT algorithms. The performance of both models was problematic, perhaps because of changes in internet search behavior and differences in the seasonality, geographical heterogeneity and age-distribution of the epidemics between the periods of GFT model-fitting and prospective use. We conclude that GFT data may not provide reliable surveillance for seasonal or pandemic influenza and should be interpreted with caution until the algorithm can be improved and evaluated. Current internet search query data are no substitute for timely local clinical and laboratory surveillance, or national surveillance based on local data collection. New generation surveillance systems such as GFT should incorporate the use of near-real time electronic health data and computational methods for continued model-fitting and ongoing evaluation and improvement.

Figure 1. Time-series of weekly influenza-like illness (ILI) surveillance and Google Flu Trends (GFT) search query estimates, June 2003–March 2013.

Observed weekly ILI proportions (black lines) are shown with Serfling model baseline (gray lines) and 95% epidemic threshold (dashed lines). The periods of the early wave of the 2009 pandemic and the 2012/2013 epidemic are shaded in grey. Sentinel ILI-Net surveillance is shown for (A) the United States and (B) Mid-Atlantic States (New Jersey, New York, Pennsylvania). Local ILI surveillance from emergency department visits is shown for (C) New York City. Scaled GFT internet search query estimates are shown for model-fitting periods for the original (thin red line) and updated (thin blue line) GFT models, and for the periods of prospective operation of the original (thick red line) and updated (thick blue line) GFT models. For Mid-Atlantic States the updated GFT model data represents ILI proportions only for New Jersey and New York (see Supporting Information).

Figure 2. Scatter plots of weekly excess influenza-like illness (ILI) visit proportions against original Google Flu Trends (GFT) model search query estimates, 2003–2009.

Weekly excess percent-ILI is calculated as Serfling estimates subtracted from observed proportions. Plots show original GFT model estimates compared with weighted CDC ILI-Net data for (A) the United States, and (B) Mid-Atlantic Census Region States (New Jersey, New York, Pennsylvania), and local ILI surveillance from emergency department visits for (C) New York City. Plots are shown for pre-pandemic influenza seasons, June 1, 2003 to April 25, 2009 (grey circles) and the early wave of the A/H1N1 pandemic, April 26 to August 1, 2009 (red diamonds). Lines representing equivalent axes for X = Y are shown (grey dashed line). Regression lines are shown for seasonal influenza 2003–2009 (black line) and the early 2009 wave of the pandemic (red line).

Figure 3. Scatter plots of weekly excess influenza-like illness (ILI) visit proportions against updated Google Flu Trends (GFT) model search query estimates, 2003–2013.

Weekly excess percent-ILI is calculated as Serfling estimates subtracted from observed proportions. Plots show updated GFT model estimates compared with weighted CDC ILI-Net data for (A) the United States, and (B) Mid-Atlantic HHS-2 Region States (New Jersey, New York), and local ILI surveillance from emergency department ILI visit data for (C) New York City. Plots are shown for weeks June 1, 2003 to April 25, 2009 (grey circles), April 26 to January 2, 2010 (red diamonds), January 3, 2010 to Oct 6, 2012 (grey squares), and October 7, 2012 to March 30, 2013 (blue triangles). Lines representing equivalent axes for X = Y are shown (grey dashed line). Regression lines are shown for the 2003/2004–2008/2009 seasons (black line), 2009 pandemic (red line), 2010/2011–2010/2012 seasons (grey solid line) and the 2012/2013 season (blue line).