Global burden of influenza: what we know and what we need to know

While influenza morbidity and mortality are now recognized as a major concern in temperate climates, this has not been the case until recently in the tropics and subtropics. Pandemics represent a special situation. Historically, the United States and Western Europe have been following influenza mortality systematically since the influenza pandemic starting in 1889. High mortality in the 1918 pandemic was well documented in these countries, but also in much of the developing world. This was also true during the 1957 pandemic, though with lower mortality. A problem for tropical countries in quantifying interpandemic influenza is the differing seasonality of outbreaks in these areas. Seasonality has been a necessary tool in such quantification. Recently, following better definition of seasonal occurrence, data have been developing on the burden of influenza in many parts of the tropics and subtropics, indicating that mortality and morbidity are likely to be similar to that identified elsewhere. Further confirmation of the impact of influenza, often underestimated in the past globally, is still needed. Selected studies need to be carried out to define health burden where this has not been done in the past. Data from these studies can be used to determine economic impact in different health care systems. (C) 2003 Elsevier B.V. All rights reserved.