Table 2. H3N2 risk level forecasts for the United States based on the cluster model.

Seasonal risk level for H3N2 influenza virus is defined as high or low for each season compared to a reference level defined as the median of the seasonal total H3N2 incidence cases in the corresponding training data set. We defined an observed season as H3N2 high risk, when the observed total H3N2 incidence surpasses the reference level; and an H3N2 low-risk season otherwise. For the forecasts, the percentage of 1000 simulations that exhibit an H3N2 high risk was obtained. When this percentage exceeded 40% (a level chosen based on fig. S5), we forecasted an H3N2 high-risk season. Otherwise, an H3N2 low-risk season was predicted.

SeasonsObserved% High
(1000 simulations)
Forecasts
(>40% high)
2011/2012Low8.2Low
2012/2013High99.6High
2013/2014Low3.6Low
2014/2015High99.9High
2015/2016Low7.0Low
2016/2017High*100.0High

*On the basis of the updated data from the weekly U.S. influenza surveillance report until week 14 ending on 8 April 2017.