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FOUNDATION OVERVIEW

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Influenza is a viral infection that attacks your respiratory system (nose, throat, and lungs) causing significant morbidity and mortality, particularly among children and elderly. Influenza occurs at any time during the year, but the highest rates occur between December and March. Influenza A and B are the two types of influenza viruses, and influenza A is further categorized into subtypes based upon two surface antigens (hemagglutinin and neuraminidase). Immunity to influenza occurs from antibody development directed at the surface antigens.

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Influenza transmission is person-to-person via inhalation of respiratory droplets with an average incubation period of 2 days (ranges 1 and 4). Classic signs and symptoms of influenza include rapid onset of fever, myalgia, headache, malaise, nonproductive cough, sore throat, and rhinitis. Nausea, vomiting, and otitis media are commonly reported in children. Signs and symptoms resolve in 3 to 7 days; however, cough and malaise may persist for more than 2 weeks. Influenza increases the risk for pneumonia (including methicillin-resistant Staphylococcus aureus pneumonia). The gold standard for the diagnosis of influenza is viral culture, but the clinical utility of the culture is limited because of the length of time to receive results. Tests such as the rapid antigen and point-of-care (POC) tests, direct fluorescence antibody (DFA) test, and the reverse transcriptase-polymerase chain reaction (RT-PCR) assay may be used for rapid detection of the influenza virus.

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PREVENTION

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Annual vaccination is the most effective method to prevent influenza; unfortunately, the annual rate of vaccination is 30% to 40%. Vaccination should be administered to any person who wishes to reduce the likelihood of becoming ill with influenza or transmitting influenza to others. While it is important for all to receive influenza vaccination, emphasis should be placed upon vaccinating groups at higher risk of influenza infection and influenza-related complications (Table 32-1). The inactivated and live-attenuated formulations are the commercially available vaccines. The inactivated vaccines are trivalent or quadrivalent, whereas the live-attenuated vaccine is quadrivalent. Most trivalent vaccines are grown in hens eggs and contain equivalent strains (influenza A H1N1, influenza A H3N2, and influenza B). The quadrivalent vaccines are grown in an identical manner and contain strains of influenza A H1N1, influenza A H3N2, and two influenza B strains. See Table 32-2 for a comparison of influenza vaccines.

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TABLE 32-1Target Groups for Vaccination

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