Characterizing the role of absolute humidity on influenza transmission and seasonality in a subtropical city, Hong Kong

Abstract

Background: Unlike temperate regions, influenza virus epidemics can have weak seasonality in tropical and subtropical locations. The underlying mechanisms of influenza seasonality remain difficult to disentangle, particularly in the tropics, where several drivers simultaneously modulate influenza seasonality. Ambient absolute humidity is one of such potential extrinsic drivers, perhaps because of its correlation with indoor relative humidity, and our aim was to characterize the role of absolute humidity on influenza transmission in Hong Kong. Methods: We analyzed surveillance data on influenza virus activity and meteorological factors in Hong Kong during 1998-2017. We used a branching process model to estimate the instantaneous reproduction number (R_t), a real-time measure of transmissibility, which is highly driven by depletion of susceptibles (S_t) along with extrinsic drivers. Hence, we estimated the transmission rate beta_t = R_t / S_t and investigated the underlying association between beta_t and extrinsic factors through multivariable nonlinear regression approach and quantify the influence of various extrinsic factors on transmissibility. Results: We identified a total of 54 distinct influenza seasons for different influenza type/subtypes over 20 years (1998-2017). We found a non-linear U-shape association between transmissibility and absolute humidity across all influenza type/subtypes in Hong Kong. The improved model could explain 10%-17% (for seasonal influenza) and 5% (for pandemic influenza) of variance in transmissibility by absolute humidity over the basic model. Sensitivity analysis indicated the fact that whole population was not initially susceptible during each epidemic. Conclusions: The non-linear (U-shape) effect of humidity on influenza transmission may contribute to the distinct irregular patterns of influenza seasonality observed in subtropical areas, including the occurrence of summer epidemics.