Influenza occurs in distinct outbreaks of varying extent every year. This epidemiologic pattern reflects the changing nature of the antigenic properties of influenza viruses, and their subsequent spread depends upon multiple factors, including transmissibility of the virus and the susceptibility of the population. Influenza A viruses, in particular, have a remarkable ability to undergo periodic changes in the antigenic characteristics of their envelope glycoproteins, the hemagglutinin and the neuraminidase.
Influenza hemagglutinin is a surface glycoprotein that binds to sialic acid residues on respiratory epithelial cell surface glycoproteins. This interaction is necessary for the initiation of infection. After viral replication, progeny virions are also bound to the host cell. Neuraminidase cleaves these links and liberates the new virions; it also counteracts hemagglutinin-mediated self-aggregation entrapment in respiratory secretions.
Among influenza A viruses that infect humans, three major subtypes of hemagglutinins (H1, H2, and H3) and two subtypes of neuraminidases (N1 and N2) have been described. Influenza B viruses have a lesser propensity for antigenic changes, and only antigenic drifts in the hemagglutinin have been described.
The epidemiology of influenza, including morbidity and mortality, will be reviewed here. The clinical manifestations, complications, diagnosis, prevention, and treatment of this infection are discussed separately; the epidemiology of pandemic H1N1 influenza ("swine influenza") and avian influenza are also presented elsewhere. (See "Clinical manifestations of seasonal influenza in adults" and "Seasonal influenza in children: Prevention and treatment with antiviral drugs" and "Seasonal influenza vaccination in adults" and "Diagnosis of seasonal influenza in adults" and "Treatment of seasonal influenza in adults" and "Prevention of seasonal influenza with antiviral drugs in adults" and "Epidemiology of pandemic H1N1 influenza ('swine influenza')" and "Epidemiology, transmission, and pathogenesis of avian influenza" and "Avian influenza A H7N9: Epidemiology, clinical manifestations, and diagnosis".)
DEFINITION OF ANTIGENIC SHIFTS AND DRIFTS
Major changes in the envelope glycoproteins, the hemagglutinin and the neuraminidase, are referred to as antigenic shifts, and minor changes are called antigenic drifts. Antigenic shifts are associated with epidemics and pandemics of influenza A, whereas antigenic drifts are associated with more localized outbreaks of varying extent.