ABSOLUTE HUMIDITY AND THE TRANSMISSION AND SEASONALITY OF INFLUENZA

The hypothesis that absolute humidity modulates influenza survival and transmission in temperate regions is investigated using laboratory and epidemiological data and a range of statistical and modeling techniques. The balance of this experimental and epidemiologic evidence indicates that variations of absolute humidity affect the onset and seasonal cycle of epidemic influenza in temperate regions. In addition, model simulations suggest that variations of the basic and effective reproduction numbers for influenza, caused by seasonal changes in absolute humidity, are also consistent with the general timing of pandemic influenza outbreaks observed for 2009 A/H1N1 in temperate regions. Indeed, absolute humidity conditions correctly identify the region of the United States vulnerable to a third, wintertime wave of pandemic influenza. These findings suggest that the timing of pandemic influenza outbreaks is controlled by a combination of absolute humidity conditions, levels of susceptibility, and changes in population-mixing and contact rates.