Absolute humidity and influenza transmissibility in subtropical Hong Kong

Abstract

Background: The seasonality of influenza virus transmission in humans largely determines the timing and frequency of annual influenza vaccination programs worldwide. However the mechanisms underlying influenza seasonality remain difficult to disentangle, particularly in temperate locations where many possible driving factors occur simultaneously in the winter. Methods: We analyzed surveillance data on influenza virus activity in a subtropical city Hong Kong during the period 1998-2013. The time varying transmissibility of influenza at the start of an epidemic and then during an epidemic was characterized by the effective (or instantaneous) reproduction number, R_t, defined as the average number of secondary infections caused by a typical single infectious individual at time t. We used mechanistic models to quantify the influence of intrinsic and extrinsic factors on the effective reproduction number. Results: Point estimates of the effective reproductive numbers at the start of each influenza epidemic fell in the range 1.2 to 1.5. We found that a large part of the variance in transmissibility (24%-55%) was explained by the depletion of susceptibles during epidemics, while 4%-7% was explained by inter-seasonal effects, 1%-3% by absolute humidity, and 2%-5% by school holidays. A strong U-shaped effect of absolute humidity was identified on influenza transmissibility although the overall association between absolute humidity and influenza transmission was comparatively small. Conclusions: The U-shaped 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.