Influenza is a highly contagious viral infection of the upper respiratory tract that is characterized by the antigen variability, the seasonality and the impact on the general population. Of the two main (A and B) types of influenza viruses, Influenza A subtypes are differenciated by antigen variability of the surface glycoproteins (hemagglutinin (15 subtypes) and neuraminidase (9 subtypes)). Influenza A are the most prevalent and are associated with the most serious epidemics. Influenza B virus usually causes a minor illness, but it does have the potential to cause more severe disease in older persons. The inability to provide lasting protection to humans against influenza B virus infection is due, in part, to antigenic drift to the viral surface glycoprotein, haemagglutinin (HA).

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