Seasonal forces and influenza virus transmission dynamics in Hong Kong

Abstract

BACKGROUND: In temperate regions influenza virus epidemics occur annually, with a peak in transmission occurring in the winter. In tropical and subtropical locations, however, influenza virus epidemics can have weak seasonality, with peaks in influenza activity not limited to the coolest and driest periods of the year. Along with depletion of susceptibles, several other factors have been proposed to cause seasonal changes in transmissibility, such as human behavior, host susceptibility, school holidays, minimum temperature and relative humidity and absolute humidity. We analyzed surveillance data on influenza virus activity in Hong Kong during the period 1998-2013, using mechanistic models to quantify the influence of intrinsic and extrinsic factors on transmission. METHOD: We used surveillance data on local influenza activity, and other local data on meteorological parameters and school holidays. We used a branching process model to estimate the daily reproductive number Rt, defined as the average number of secondary infections caused by a typical single infectious individual at time t. We used Gamma-distributed serial intervals with means of 3.0-3.7 days for each influenza type/subtype. We then used multiple linear regression models to investigate the underlying association between the Rt and various intrinsic and extrinsic factors. RESULTS: We identified a total of 38 distinct influenza seasons (8 seasons for seasonal H1N1, 3 seasons for H1N1pdm09, 16 seasons for seasonal H3N2 and 11 seasons for seasonal B) over the 16 years from January 1998 through December 2013. Point estimates of Rt at the start of each epidemic ranged from 1.20 to 1.50, with no clear differences between influenza types/subtypes, while the highest single value was 1.50 for the start of the first wave of H1N1pdm09. We found most of the observed variance in the inferred reproduction numbers Rt was explained by the depletion of susceptibles during the outbreak, inter-seasonal effects, school holidays, and absolute humidity. CONCLUSION: The seasonal influenza in tropical and subtropical regions are highly irregular compare to that of temperate regions such as Europe and North America. The instantaneous reproduction number Rt had a weakly but highly statistically significant association with absolute humidity and school holidays, which might influence the timing of influenza epidemics.