|Title||Global environmental drivers of influenza|
|Publication Type||Journal Article|
|Year of Publication||2016|
|Authors||Deyle ER, Maher M.C, Hernandez R.D, Basu S., Sugihara G|
|Journal||Proceedings of the National Academy of Sciences of the United States of America|
|Type of Article||Article|
|Keywords||causality; chaos; empirical dynamic modeling; epidemiology; error; nonlinear dynamics; physical-biological coupling; relative-humidity; seasonality; state-dependence; survival; temperature; time-series; transmission; virus|
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 degrees F. The results indicate a unified explanation for environmental drivers of influenza that applies globally.