Spatial-temporal variations of influenza activity in two subtropical cities

Abstract

BACKGROUND: The influenza virus causes seasonal epidemics in humans every year and has been associated with heavy disease burden around the world. One mystery about this virus is its distinct seasonal pattern across different climates. In temperate regions, influenza usually exhibits one winter peak, whereas influenza peaks are less clearly defined in tropical and subtropical regions. It has been proposed that virulence of the virus, environmental factors, and human activities are the major variables that determine influenza seasonality. In this study, we assessed the spatial and temporal variations of influenza activity across different districts in 2 adjacent subtropical cities, Shenzhen and Hong Kong, with the aim to explore the impact of environmental factors and human activities on influenza seasonality. Shenzhen is one of the largest migratory metropolitan cities in mainland China and has 4 districts located at the special economic zone and 2 suburban districts, the latter of which host most factories and immigrant workers. These districts show great heterogeneity in terms of living environment, socioeconomic development, and frequency of cross-border communications with the adjacent international metropolitan Hong Kong. METHODS: Influenza activity was measured by weekly consultation rate of influenza-like illness (ILI) and weekly proportion of specimens positive for influenza virus A or B from 2006 to 2011. These data were collected from general hospitals and community health centers in Shenzhen and general outpatient clinics and general practitioners in Hong Kong. To tackle the nonstationary aspect of both ILI and virology data, we applied wavelet analysis to examine the association between ILI consultation rates and virus activity. The lag period between ILI and virology data within and across different districts in Shenzhen and Hong Kong was assessed by phase analysis. RESULTS: ILI consultation rates in 6 districts of Shenzhen and 4 regions of Hong Kong exhibited annual cycles during the study period, except that a semiannual cycle appeared in some districts of Shenzhen after the 2009 pandemic. We found temporal synchrony in the virology data between Shenzhen and Hong Kong and between the ILI consultation rates and virology data across the different districts (or regions) within each city. But this synchrony was not shown in most of the Shenzhen districts after 2009. The Nanshan district of Shenzhen and the New Territories West region of Hong Kong preceded other districts (or regions) by weeks in the oscillation of ILI consultation rates, particularly during the 2009 influenza pandemic. There was no obvious difference between the special economic zone and suburban districts of Shenzhen in terms of temporal variations. CONCLUSIONS: Regional heterogeneity in influenza virus activity was observed across the different districts of Shenzhen and Hong Kong, which may not be explained by regional difference in socioeconomic development and human behavior. Further studies are needed to confirm the regional source of influenza seasonal outbreaks in this subtropical area.