Unified climate factors predict influenza outbreak seasonality across tropical and temperate regions

Influenza represents a source of considerable morbidity and mortality worldwide, exhibiting seasonal outbreak patterns that vary by location. Temperate regions typically experience sharp wintertime peaks, while tropical regions tend to exhibit lower-intensity, year-round activity, sometimes with two outbreak peaks a year. A key question is how a common set of factors interact to produce distinct outbreak patterns across temperate and tropical locations. Here, we compile a novel dataset of influenza surveillance data across North and South America and fit a mechanistic susceptible-infected-recovered-susceptible model driven by locally resolved climate data to test whether a unified set of climate variables can explain regional differences in influenza outbreak dynamics. We find that a combined, nonlinear effect of specific humidity and temperature can explain the seasonality of outbreaks across both temperate and tropical climate regimes. Leveraging this model, we explore the potential impacts of climate change on influenza outbreaks. We find that while temperate areas may experience a decline in peak size, outbreak intensity in tropical areas could increase under climate change.