Abstract
Laboratory and epidemiological evidence indicate that ambient humidity modulates the survival and transmission of influenza. Here we explore whether the inclusion of humidity forcing in mathematical models describing influenza transmission improves the accuracy of forecasts generated with those models. We generate retrospective forecasts for 95 cities over 10 seasons in the United States and assess both forecast accuracy and error. Overall, we find that humidity forcing improves forecast performance (at 1–4 lead weeks, 3.8% more peak week and 4.4% more peak intensity forecasts are accurate than with no forcing) and that forecasts generated using daily climatological humidity forcing generally outperform forecasts that utilize daily observed humidity forcing (4.4% and 2.6% respectively). These findings hold for predictions of outbreak peak intensity, peak timing, and incidence over 2- and 4-week horizons. The results indicate that use of climatological humidity forcing is warranted for current operational influenza forecast.
Highlights
A growing body of evidence indicates that the survival and transmissibility of influenza are affected by ambient humidity conditions
These findings were confirmed by Friedman rank, which showed that overall the no absolute humidity (AH) forecasts ranked the worst among the four AH approaches (Table 1)
Our findings indicate that peak intensity forecast accuracy improves with some type of AH forcing regardless of lead time
Summary
A growing body of evidence indicates that the survival and transmissibility of influenza are affected by ambient humidity conditions. Laboratory experiments have shown that aerosolized influenza survival rates increase in low ambient relative humidity (RH), i.e. less than 40% [1,2,3,4,5,6,7,8,9,10]. Additional experiments examining the transmission of human influenza among guinea pigs have shown that transmission increases at low RH levels [11]. Further evaluation of these experiments has revealed a strong relationship in which low absolute humidity (AH) conditions favor the survival and transmission of influenza [12]. Even though RH is often maximal outdoors during winter, indoor RH often reaches the very low levels (10–40%) favorable for influenza survival and transmission, and indoor AH closely mirrors outdoor levels [13,14,15,16]
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