Antigenic shift

Antigenic shift is the process by which two different strains of influenza combine to form a new subtype which is characterized by a mixture of the surface antigens of the two original strains. Because the human immune system has difficulty recognizing the new influenza strain, such a new subtype may be highly dangerous.

This is contrasted with antigenic drift which is the natural mutation over time of known strains of influenza (or other things, in a more general sense) to evade the immune system.

While antigenic drift occurs in all types of influenza including influenza A, B and C, antigenic shift only occurs in influenza A because only influenza A infects humans as well as animals and thus has the opportunity for a major reorganization of surface antigens. Influenza B and C only infect humans.

Flu strains are named after their types of hemagglutinin and neuraminidase surface proteins, so they will be called, e.g. H3N2 for type-3 hemagglutinin and type-2 neuraminidase.

If two different strains of influenza infect the same cell simultaneously, their protein capsids and lipid envelopes are removed, exposing their RNA, which is then transcribed to DNA. The host cell then forms new viruses that combine antigens, e.g. H3N2 and H5N1 can form H5N2 this way. This has happened four times in the twentith century. Once in 1918 in the infamous "Spanish Flu" outbreak which killed 40 million people worldwide. Influenza virus which have undergone antigenic shift have also gone on to cause the "Asian Flu" pandemic of 1957, the "Hong Kong Flu" pandemic of 1968, and the "Swine Flu" scare of 1976.

In 2004, scientists pointed out that the bird flu virus might undergo an antigenic shift with the human flu virus and cause a global influenza pandemic like the one in 1918.

The term antigenic shift is specific to the influenza literature. In other viral systems, the same process is called reassortment.