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Influenza is an acute respiratory illness caused by infection with influenza viruses. Most people with influenza usually recover within one week, although cough and fatigue may persist longer. However, some people are more prone to developing complications that can result in hospitalizations and, occasionally, death. Potential complications include bacterial sinusitis, secondary bacterial pneumonia, otitis media and (rarely) viral pneumonia, respiratory failure and exacerbations of underlying disease.
Influenza-related complications that require urgent medical care can arise as a result of the specific factors associated with age or pregnancy, direct effect of the influenza virus on the human organism, or chronic diseases such as various cardiopulmonary conditions. However, studies that have measured rates of a clinical outcome without a laboratory confirmation of influenza virus infection are often difficult to interpret, due to the concomitant circulation of other respiratory pathogens (e.g. respiratory syncytial virus) at the same time.
It is estimated that annual epidemics of influenza are responsible for up to 5 million cases of severe illness and 250 000 to 500 000 deaths worldwide. The risk of hospitalization is highest in people aged 65 years or older, in young children, and in people with chronic medical conditions.
More than 90% of influenza-related deaths during recent seasonal epidemics in the United States have been in people aged 65 years or older. In developing countries, a large proportion of child deaths are associated with influenza disease.
Longer duration of symptoms, positive markers of inflammation and coagulation, and immunosuppression are associated with an increased risk of disease progression. A myriad of other factors show significant association with severity of disease that include underlying chronic medical conditions, neurological diseases, morbid obesity and pregnancy.
Patients with influenza are thought to be at higher risk for secondary bacterial infection and pneumonia due to the cytopathic effects of viral replication in cells, as well as dysregulated changes in host cytokine production that may dampen both the ability of the immune system to clear bacteria and to accomplish appropriate modulation of the inflammatory cascade.
The preponderance of data links smoking to an increased incidence and severity of respiratory bacterial and viral infections – including influenza. This association is particularly pronounced among heavy smokers, and animal studies are already conducted that confirm this association.
Cases of Guillain-Barré syndrome (the most common cause of acute flaccid paralysis) associated with influenza viruses appear to constitute a specific entity. Most cases of this syndrome happen as a result of an autoimmune response triggered by a recent infectious disease or vaccination. Influenza-related Guillain-Barré syndrome occurs relatively infrequently, with an incidence of only 4–7 cases per 100,000 influenza cases.
The best available method for prevention of influenza is vaccination. In humans, the seasonal influenza vaccine is developed to protect against endemic H1N1, H3N2 and B strains in global circulation. The conventional influenza vaccine model is based on stimulating immunity against the major neutralizing antibody target, hemagglutinin (HA), by virus inactivation or attenuation.
The goals of influenza vaccination can broadly be defined as protection against infection and complications, as well as the induction of herd immunity which hinders viral transmission within the population. The success of influenza vaccination campaigns depends on extensive surveillance and manufacturing resources in order to ensure timely vaccine delivery.
Guidelines for influenza vaccination vary significantly among countries. In the United States, annual influenza vaccination programs are recommended for high-risk groups since the 1960s, but in 2010 the Advisory Committee on Immunization Practices made the first recommendation for a national influenza vaccination to include individuals between 6 months and 18 years of age. In Europe, the majority of countries still generally recommend vaccination only for the chronically ill, elderly and residents of long-term health care facilities.
The effectiveness of inactivated trivalent influenza vaccine varies in different age groups, but general rule is that it is less effective among the elderly than in the young population. Since influenza vaccines do not induce long-lasting antibody titers, annual influenza vaccination is recommended before the start of the winter season.
In order to achieve maximal effectiveness of the vaccine, influenza virus strains have to be antigenically matched to the viruses circulating in humans. The most appropriate vaccine strains are identified by a complex and extensive global surveillance effort, which is coordinated by the World Health Organization (WHO) via its Global Influenza Surveillance and Response System network.