Influenza

Cohen, Cheryl; Reubenson, Gary

Influenza is one of the commonest infections in human populations, and causing substantial morbidity and mortality globally. The influenza virus is divided into different types and subtypes, three of which are currently circulating widely in humans: influenza A(H3N2) and influenza B. The virus undergoes constant evolution, leading to annual seasonal winter epidemics in temperate countries and necessitating annual updates to the vaccine. Rarely, completely new influenza viruses can emerge in human populations, giving rise to influenza pandemics. Children aged <5 years (especially those <2 years) and those with underlying illness such as cardiac, respiratory and severe neurologic disease have an increased risk of severe outcomes associated with influenza. Pregnant women have an increased risk of severe influenza. Complications may involve the respiratory tract (e.g. otitis media or pneumonia) or, less commonly, other organ systems (e.g. encephalitis or myocarditis). Specific antiviral treatment should be offered as soon as possible for hospitalized children with presumed or confirmed influenza and for influenza of any severity for children at high risk of severe complications of influenza without waiting for laboratory confirmation. Antiviral treatment is usually not warranted for uncomplicated influenza as this is usually self-limiting. Annual influenza vaccination should be offered to all individuals at increased risk for complications of influenza. Vaccine cannot be given to children aged <6 months but maternal influenza immunization during pregnancy is recommended and can confer protection to the young infant.