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Hospitalizations and Deaths In the United States, annual epidemics of influenza typically occur during the fall or winter months, but the peak of influenza activity can occur as late as April or May (Table 1). Influenza-related hospitalizations or deaths can result from the direct effects of influenza virus infection or from complications due to underlying cardiopulmonary conditions and other chronic diseases. Studies that have measured rates of a clinical outcome without a laboratory confirmation of influenza virus infection (e.g., respiratory illness requiring hospitalization during influenza season) to assess the effect of influenza can be difficult to interpret because of circulation of other respiratory pathogens (e.g., respiratory syncytial virus) during the same time as influenza viruses (46-48). During seasonal influenza epidemics from 1979-1980 through 2000-2001, the estimated annual overall number of influenza-associated hospitalizations in the United States ranged from approximately 55,000 to 431,000 per epidemic (mean: 226,000); the estimated annual number of deaths attributed to influenza ranged from 8,000 to 68,000 per epidemic (mean: 34,000) (1,2). Since the 1968 influenza A (H3N2) virus pandemic, the number of influenza-associated hospitalizations typically has been greater during seasonal influenza epidemics caused by type A (H3N2) viruses than during seasons in which other influenza virus types or subtypes have predominated (49). In the United States, the number of influenza-associated deaths has increased since 1990. This increase has been attributed in part to the substantial increase in the number of persons aged >65 years, who are at increased risk for death from influenza complications (50). In one study, an average of approximately 19,000 influenza-associated pulmonary and circulatory deaths per influenza season occurred during 1976-1990, compared with an average of approximately 36,000 deaths per season during 1990-1999 (1). In addition, influenza A (H3N2) viruses, which have been associated with higher mortality (51), predominated in 90% of influenza seasons during 1990-1999, compared with 57% of seasons during 1976-1990 (1). Influenza viruses cause disease among persons in all age groups (3-5). Rates of infection are highest among children, but the risks for complications, hospitalizations, and deaths from influenza are higher among persons aged >65 years, young children, and persons of any age who have medical conditions that place them at increased risk for complications from influenza (1,3,6-8,52-55). Estimated rates of influenza-associated hospitalizations and deaths varied substantially by age group in studies conducted during different influenza epidemics (Table 2). During 1990-1999, estimated rates of influenza-associated pulmonary and circulatory deaths per 100,000 persons were 0.4-0.6 among persons aged 0-49 years, 7.5 among persons aged 50-64 years, and 98.3 among persons aged >65 years (1). |