Spatial Variation in Humidity and the Onset of Seasonal Influenza Across the Contiguous United States

In recent years, environmental factors, particularly humidity, have been used to inform influenza prediction models. This study aims to quantify the relationship between humidity and influenza incidence at the state-level in the contiguous United States. Piecewise segmented regressions were performed on specific humidity data from NASA's Atmospheric Infrared Sounder (AIRS) and incident influenza estimates from Google Flu Trends to identify threshold values of humidity that signal the onset of an influenza outbreak. Our results suggest that influenza incidence increases after reaching a humidity threshold that is state-specific. A linear regression showed that the state-specific thresholds were associated with annual average humidity conditions (R-2 = 0.9). Threshold values statistically significantly varied by region (F-statistic = 8.274, p < 0.001) and of their 36 pairwise combinations, 13 pairs had at least marginally statistically significant differences in their means. All of the significant comparisons included either the South or Southeast region, which had higher humidity threshold values. Results from this study improve our understanding of the significance of humidity in the transmission of influenza and reinforce the need for local and regional conditions to be considered in this relationship. Ultimately this could help researchers to produce more accurate forecasts of seasonal influenza onset and provide health officials with better information prior to outbreaks. Plain Language Summary The influenza, or flu, virus is a contagious respiratory illness that infects millions of people in the United States each year. Scientists from multiple disciplines have been using complex models to try and predict the start of seasonal outbreaks using a variety of information. Humidity has been shown in laboratory experiments to be a potentially important component of influenza transmission. This study uses historical estimates of influenza case numbers as well as humidity data to investigate the relationship between the two at the state-level across the contiguous United States. We found the humidity values that seemed to signal the onset of seasonal influenza differed by state and that in states with higher average annual humidity, the humidity value that "signaled" seasonal onset was also higher. Additionally, we found that there were regional patterns in our results. This work could improve our understanding of how humidity impacts influenza transmission and how we use humidity in models that aim to predict seasonal outbreaks.