title: Global environmental drivers of influenza.
abstract: In temperate countries, influenza outbreaks are well correlated to seasonal changes in temperature and absolute humidity. However, tropical countries have much weaker annual climate cycles, and outbreaks show less seasonality and are more difficult to explain with environmental correlations. Here, we use convergent cross mapping, a robust test for causality that does not require correlation, to test alternative hypotheses about the global environmental drivers of influenza outbreaks from country-level epidemic time series. By moving beyond correlation, we show that despite the apparent differences in outbreak patterns between temperate and tropical countries, absolute humidity and, to a lesser extent, temperature drive influenza outbreaks globally. We also find a hypothesized U-shaped relationship between absolute humidity and influenza that is predicted by theory and experiment, but hitherto has not been documented at the population level. The balance between positive and negative effects of absolute humidity appears to be mediated by temperature, and the analysis reveals a key threshold around 75 °F. The results indicate a unified explanation for environmental drivers of influenza that applies globally.
authors: Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093.
authors: Ethan R
authors: ER
authors: Deyle
authors: Human Longevity Inc., Mountain View, CA 94041.|Department of Epidemiology and Translational Science, University of California, San Francisco, CA 94158.
authors: M Cyrus
authors: MC
authors: Maher
authors: Department of Bioengineering and Therapeutic Sciences, University of California, San Francisco, CA 94158.
authors: Ryan D
authors: RD
authors: Hernandez
authors: Stanford Prevention Research Center, Stanford University School of Medicine, Stanford, CA 94305.
authors: Sanjay
authors: S
authors: Basu
authors: Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093; gsugihara@ucsd.edu.
authors: George
authors: G
authors: Sugihara
mesh_terms: D004196:Disease Outbreaks; D006801:Humans; D006813:Humidity; D007251:Influenza, Human / Q000453:epidemiology*; D012621:Seasons; D013696:Temperature
keywords: empirical dynamic modeling; epidemiology; nonlinear dynamics; physical–biological coupling; state-dependence