Epidemiological aspects of 2009 H1N1 influenza: the accumulating experience from the Northern Hemisphere

Preliminary data regarding the experience of countries of the Northern Hemisphere with pandemic 2009 A(H1N1) influenza have already appeared in the literature. We aimed to evaluate the available published literature describing the epidemiological features of pandemic influenza. We searched PubMed; 35 studies (14 referred to European countries, eight to the USA, five to Mexico, four to Canada, two to Japan, one to Colombia, and one reviewed relevant data reported worldwide) were included. Considerably high hospitalization, intensive care unit (ICU) admission, and fatality rates (up to 93.8, 36.4, and 38.5%, respectively) among the evaluated cases were reported across studies with available relevant data. Young and middle-aged adults constituted the majority of the evaluated pandemic cases, with different disease severity (as indicated by the level of care and outcome). Yet, substantial percentages of elderly individuals were reported among more severely afflicted cases. Otherwise healthy patients constituted substantial percentages among evaluated cases with different disease severity. Pregnant women, obese, and morbidly obese patients also constituted substantial percentages of the cases involved in the included studies. The evaluation of the currently available published evidence contributes to the clarification of the epidemiological features of pandemic 2009 A(H1N1) influenza, which is useful in terms of the individual and public health perspectives.