The number of pages within the document is: 27
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Public Health England, Immunisation Department
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2019-02-07 23:00:32.880522
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119Influenza The diseaseInfluenza is an acute viral infection of the respiratory tract. There are three types of influenza virus: A, B and C. Influenza A and influenza B are responsible for most clinical illness. Influenza is highly infectious with a usual incubation period of one to three days. The disease is characterised by the sudden onset of fever, chills, headache, myalgia and extreme fatigue. Other common symptoms include a dry cough, sore throat and stuffy nose. For otherwise healthy individuals, influenza is an unpleasant but usually self-limiting disease with recovery usually within two to seven days. The illness may be complicated by (and may present as) bronchitis, secondary bacterial pneumonia or, in children, otitis media. Influenza can be complicated more unusually by meningitis, encephalitis or meningoencephalitis. The risk of serious illness from influenza is higher amongst children under six months of age (Poehling et al., 2006; Ampofo et al., 2006; Coffin et al., 2007; Zhou et al, 2012), older people (Thompson et al., 2003 and 2004; Zhou et al, 2012) and those with underlying health conditions such as respiratory or cardiac disease, chronic neurological conditions, or immunosuppression and pregnant women (Neuzil et al., 1998; O™Brien et al., 2004; Nicoll et al., 2008 and Pebody et al., 2010). Influenza during pregnancy may also be associated with perinatal mortality, prematurity, smaller neonatal size and lower birth weight (Pierce et al., 2011; Mendez-Figueroa et al., 2011). Although primary influenza pneumonia is a rare complication that may occur at any age and carries a high case fatality rate (Barker and Mullooly, 1982), it was seen more frequently during the 2009 pandemic and the following influenza season. Serological studies in healthcare professionals have shown that approximately 30 to 50% of influenza infections can be asymptomatic (Wilde et al., 1999) but the proportion of influenza infections that are asymptomatic may vary depending on the characteristics of the influenza strain.Transmission is by droplets, aerosol, or through direct contact with respiratory secretions of someone with the infection (Lau et al., 2010). Influenza spreads rapidly, especially in closed communities. Most cases in the UK tend to occur during an eight- to ten-week period during the winter. The timing, extent and severity of this ‚seasonal™ influenza can all vary. Influenza A viruses cause outbreaks most years and it is these viruses that are the usual cause of epidemics. Large epidemics occur intermittently. Influenza B tends to cause less severe disease and smaller outbreaks overall. The burden of influenza B disease is mostly in children when the severity of illness can be similar to that associated with influenza A. Changes in the principal surface antigens of influenza A Œ haemagglutinin and neuraminidase Œ make these viruses antigenically labile. Minor changes described as antigenic drift occur progressively from season to season. Antigenic shift occurs periodically, resulting in major changes and the emergence of a new subtype with a different haemagglutinin protein. Because immunity from the previous virus may not
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