Absolute humidity and temperature are the most important environmental drivers of global influenza, despite differences in outbreak patterns between tropical and temperate countries, according to a new analysis by U.S.-based researchers.
Using convergent cross-mapping and an empirical dynamic modeling approach on data collected by the World Health Organization, investigators led by George Sugihara, PhD, of the Scripps Institution of Oceanography at the University of California, San Diego, confirmed a hypothetical U-shaped relationship between influenza outbreaks and absolute humidity. At low latitudes in the tropics, absolute humidity has a positive effect, increasing the likelihood of influenza as humidity rises but at higher latitudes in temperate countries, absolute humidity has a negative effect, making influenza more likely when absolute humidity is low.
While absolute humidity was the most important factor in the likelihood of influenza outbreaks, the U-shaped relationship was dictated by average temperature. An average temperature below 70 °F had little effect on the negative relationship between absolute humidity and influenza at that range of temperatures, but if the temperature was between 75 °F and 85 °F, the effect was positive. Above 85 °F, aerosol transmission of influenza is blocked, the investigators noted.
“Augmented with further laboratory testing, these population-level results could help set the stage for public health initiatives such as placing humidifiers in schools and hospitals during cold, dry, temperate winter, and in the tropics, perhaps using dehumidifiers or air conditioners set above 75 °F to dry air in public buildings,” Dr. Sugihara and his colleagues wrote.
Find the full study in Proceedings of the National Academy of Sciences of the United States of America (doi: 10.1073/pnas.1607747113).