Influenza in Tropical Regions

April 2006 | Volume 3 | Issue 4 | e89 The global burden of infl uenza on morbidity and mortality is considerable, with an estimated 1 million annual deaths worldwide [1]. In temperate regions, there are clear seasonal variations in the occurrence of infl uenza, with a marked peak in cold winter months [2]. In contrast, seasonality is less defi ned in tropical regions—here, there is high background infl uenza activity throughout the year, on top of which epidemics occur at intermediate months between the infl uenza season in temperate countries of the Northern and Southern hemispheres. Infl uenza seasonal patterns in the Americas are summarized in Figure 1, which illustrates the transition between the Northern and Southern hemispheres. Very little is known about exactly how and why the seasonality of infl uenza varies with latitude. No link with environmental conditions has been convincingly shown, although infl uenza activity appears to coincide with the rainy season in some tropical countries [3]. Until very recently, the impact of infl uenza on health was believed to be insignifi cant in tropical regions. But this impression may be changing, in part thanks to the recent studies by Joseph S. M. Peiris and his colleagues from Hong Kong, one of which appears in this issue of PLoS Medicine [4]. Hong Kong is a subtropical city located within the likely epicenter of pandemic infl uenza in Southeast Asia [5].