Influenza - antigenic drift and shift explained.

Influenza viruses are constantly evolving by reassortment of their genetic material. Mutations cause minute changes in the hemagglutinin and neuraminidase antigens on the surface of the virus. This change is gradual(increasing with each division cycle) and is called antigenic drift. Since the strains produced by drift are somewhat similar to the older strains, some people are still immune and some others will be partly immune (and get a milder illness).

It is due to this antigenic drift that a new vaccine has to be manufactured and reimmunization is recommended each year.

In contrast, antigenic shift occurs when influenza viruses reassort. The virus acquires completely new antigens—for example by reassortment between avian strains and human strains. If a human influenza virus is produced that has entirely new antigens, everybody will be susceptible, and the novel influenza will spread uncontrollably, causing a pandemic.