The impact of the seasonal influenza and 2009 AH1N1 pandemic influenza on mortality is not yet completely understood, particularly in tropical and subtropical countries. The trends of influenza related mortality rate in different age groups and different outcomes on a area in tropical and subtropical climate with more than 41 million people (State of S？o Paulo, Brazil), were studied from 2002 to 2011 were studied. Serfling-type regression analysis was performed using weekly mortality registries and virological data obtained from sentinel surveillance. The prepandemic years presented a well-defined seasonality during winter and a clear relationship between activity of AH3N2 and increase of mortality in all ages, especially in individuals older than 60 years. The mortality due to pneumonia and influenza and respiratory causes associated with 2009 pandemic influenza in the age groups 0–4 years and older than 60 was lower than the previous years. Among people aged 5–19 and 20–59 years the mortality was 2.6 and 4.4 times higher than that in previous periods, respectively. The mortality in all ages was higher than the average of the previous years but was equal mortality in epidemics of AH3N2. The 2009 pandemic influenza mortality showed significant differences compared to other years, especially considering the age groups most affected. 1. Introduction Influenza is a significant cause of mortality in temperate countries [1–3]. There are still many questions concerning the impact of influenza on mortality in the tropics [4–6]. There is also a lot of controversy regarding the quantitative aspects of mortality associated with the 2009 AH1N1 pandemic. It is observed differing severity of disease in many region, as shown by studies conducted in Mexico, France, USA, and other countries using different methodologies [7–11]. The direct measurement of influenza-related mortality is difficult for several reasons [12]. It is a disease with very nonspecific early symptoms; moreover, physicians often do not collect specimens for diagnostic confirmation. In addition, the time period between onset of symptoms and hospitalization is often too large and does not allow a conclusive diagnosis. Another reason is that many patients die from bacterial complications or from decompensation of preexisting conditions, leading to confusion in defining the underlying cause. For these reasons, the basic cause in the death certificate is rarely influenza, although it has been the root cause of the events that led the patient to death [12, 13]. Despite facing these difficulties, the
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