Why are patients at risk vaccinated every year for influenza?

Influenza is caused by two species of orthomyxoviruses. The genome of the virus is extremely variable, especially the genes that code for hemagglutinin and neuraminidase, which form the glycoprotein spikes for these viruses. This occurs through a process known as antigenic drift and antigenic shift. Antigenic drift refers to the accumulation of HA and NA mutations within a single strain of a virus in a given geographic location. This changes the antigenicity of the virus to the point that it becomes so antigenically distinct from previous strains that no one is immune to it. Antigenic drift leads to an increase in the number of influenza cases every two years or so. An antigenic shift is a major genetic change that results when genomes from different influenza virus strains reassort in a single host. This reassortment also changes the virus's antigenicity. Antigenic shift takes place every 10 years and is responsible for most pandemic strains. Therefore, because of the antigenic change of the causative agent, patients who are at risk should receive vaccination every year with vaccines developed against antigens of newly emerging influenza viruses.