Figure 2: Accuracy of 2012–2013 real-time forecasts.

Plots comparing the weekly fraction of accurate SIRS-EAKF forecasts with the accuracy of analogue forecasts derived from historical probabilities (see Supplementary Methods). Top: weekly SIRS-EAKF forecast accuracy and resampled predictions using two alternate resampling approaches. Bottom: weekly SIRS-EAKF forecast accuracy and resampled analogue predictions based on historically observed durations between initially elevated ILI+ and peak ILI+. Only cities that have exceeded an onset, or initial threshold, level of elevated ILI+ are included in the analogue forecast for a given week. Three different onset thresholds are shown as follows: 100, 500 and 2,000 ILI+. For all the analogue forecasts, the thick line depicts the mean fraction of accurate forecasts while the shading and thin lines delineate 95% bootstrap confidence intervals.