based on the characteristics of the hemagglutinin (H or HA) and neuraminidase (N or NA) surface proteins (Treanor, 2010). This subtyping is the basis of the H#N# designations of the influenza A viruses. At least 16 distinct HA and 9 distinct NA surface proteins have been identified (Treanor, 2010). Influenza B viruses are subdivided as Yamagada or Victoria based on genetic lineage (Xu et al., 2004). Of the three distinct types of influenza viruses, influenza A viruses are the only viruses proven to cause pandemic disease and are capable of interspecies transmission, as demonstrated with the 1997 outbreak of avian (H5N1) influenza from poultry to humans (de Jong et al., 1997; Subbarao et al., 1998; Yuen et al., 1998).

Influenza viruses have caused epidemics every 1 to 3 years during the past four centuries, and four major pandemics have occurred including the great pandemic of 1918 (Treanor, 2010). These pandemics were caused by influenza A viruses H1N1 (1918 and 2009), H2N2 (1957), and H3N2 (1968) (Treanor, 2010). In any given year, two influenza A strains considered to be most likely to contribute to widespread (epidemic or pandemic) illness are included in the trivalent vaccine. Because of its ability to produce epidemic disease, an influenza B virus strain is also included in all current vaccines.

In the United States, a nearly annual influenza epidemic usually begins in late fall and peaks in mid to late winter. Influenza viruses are transmitted by contact with aerosol secretions containing the virus, and this occurs generally through coughing and sneezing (Belshe et al., 2008; Treanor, 2010). Following an average incubation period of 2 days but ranging from 1 to 4 days, adults and children remain infectious for approximately 5 days after the onset of the illness (CDC, 2002). Children, who generally have the highest attack rate and serve as the major source of transmission within communities (Glezen and Couch, 1978; Monto and Kioumehr, 1975), can be infectious for longer periods both before and after the onset of illness (Belshe et al., 2008). Uncomplicated influenza often begins abruptly with systemic symptoms of fever, chills, headaches, myalgia, malaise, anorexia, and fatigue. These symptoms persist for the duration of the fever— typically for 3 days (Treanor, 2010). Respiratory symptoms such as dry cough, sore throat, and nonproductive cough may also occur and usually persist for 2 weeks or more (Belshe et al., 2008; Treanor, 2010). Fevers tend to be higher in children and can lead to febrile seizures, while elderly individuals may experience afebrile disease with lassitude or confusion (Babcock et al., 2006; Bridges et al., 2008; Neuzil et al., 2003). The risk of complications from influenza is higher in children and the elderly and those with certain underlying conditions (Barker, 1986; Bridges et al., 2008; Simonsen et al., 2000; Thompson et al., 2004). The most common complications include primary influenza viral pneumonia, secondary bacterial pneumonia, and the



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