Absolute Humidity and the Seasonal Onset of Influenza in the Continental United States

Summary

Much of the observed wintertime increase of mortality in temperate regions is attributed to seasonal influenza. A recent reanalysis of laboratory experiments indicates that absolute humidity strongly modulates the airborne survival and transmission of the influenza virus. Here, we extend these findings to the human population level, showing that the onset of increased wintertime influenza-related mortality in the United States is associated with anomalously low absolute humidity levels during the prior weeks. We then use an epidemiological model, in which observed absolute humidity conditions temper influenza transmission rates, to successfully simulate the seasonal cycle of observed influenza-related mortality. The model results indicate that direct modulation of influenza transmissibility by absolute humidity alone is sufficient to produce this observed seasonality. These findings provide epidemiological support for the hypothesis that absolute humidity drives seasonal variations of influenza transmission in temperate regions.

Key Points

• 1Low absolute humidity is associated with increased influenza transmission during winter
• 2The model simulates influenza mortality patterns based on absolute humidity conditions
• 3Anomalous low absolute humidity precedes the onset of influenza outbreaks
• 4Statistical analyses confirm significant associations between humidity and influenza seasonality
• 5The findings could inform short-term forecasts of influenza epidemics.

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