Influenza is an infectious respiratory disease that, in humans, is caused by influenza A and influenza B viruses. Typically characterized by annual seasonal epidemics, sporadic pandemic outbreaks involve influenza A virus strains of zoonotic origin. The WHO estimates that annual epidemics of influenza result in 1 billion infections, 3-5 million cases of severe illness and 300,000-500,000 deaths. The severity of pandemic influenza depends on multiple factors, including the virulence of the pandemic virus strain and the level of pre-existing immunity. The most severe influenza pandemic, in 1918, resulted in >40 million deaths worldwide. Influenza vaccines are formulated every year to match the circulating strains, as they evolve antigenically owing to antigenic drift. Nevertheless, vaccine efficacy is not optimal and is dramatically low in the case of an antigenic mismatch between the vaccine and the circulating virus strain. Antiviral agents that target the influenza virus enzyme neuraminidase have been developed for prophylaxis and therapy. However, the use of these antivirals is still limited. Emerging approaches to combat influenza include the development of universal influenza virus vaccines that provide protection against antigenically distant influenza viruses, but these vaccines need to be tested in clinical trials to ascertain their effectiveness.

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Journal Nature Reviews Disease Primers
Krammer, F. (Florian), Smith, G.J.D. (Gavin J.D.), Fouchier, R.A.M, Peiris, M. (Malik), Kedzierska, K, Doherty, P, … García-Sastre, A. (2018). Influenza. Nature Reviews Disease Primers (Vol. 4, pp. 1–21). doi:10.1038/s41572-018-0002-y