Reassortment is the mixing of the genetic material of a species into new combinations in different individuals. Several different processes contribute to reassortment, including assortment of chromosomes, and chromosomal crossover. It is particularly used when two similar viruses that are infecting the same cell exchange genetic material. In particular, reassortment occurs among influenza viruses, whose genomes consist of eight distinct segments of RNA. These segments act like mini-chromosomes, and each time a flu virus is assembled, it requires one copy of each segment.

If a single host (a human, a chicken, or other animal) is infected by two different strains of the influenza virus, then it is possible that new assembled viral particles will be created from segments whose origin is mixed, some coming from one strain and some coming from another. The new reassortant strain will share properties of both of its parental lineages.

Reassortment is responsible for some of the major genetic shifts in the history of the influenza virus. The 1957 and 1968 pandemic flu strains were caused by reassortment between an avian virus and a human virus, whereas the H1N1 virus responsible for the 2009 swine flu outbreak has an unusual mix of swine, avian and human influenza genetic sequences.

A flu virus contains eight gene segments. One of the gene segments codes for the surface antigen hemagglutinin (HA), and another codes for the surface antigen neuraminidase (NA). Each year, researchers predict which flu strains will be most prevalent and select three – two influenza A strains and an influenza B strain – to be included in that year’s vaccine.

The goal of reassortment is to combine the desired HA and NA antigens from the target strain (flu strain 1) with genes from a harmless strain that grows well in an egg (flu strain 2).

The illustration details the following steps in creating the vaccine:

• Flu strains 1 and 2 are injected into a fertilized chicken egg.
• The genes from flu strain 1 multiply and mix with the genes from flu strain 2, forming as many as 256 possible gene combinations.
• Researchers search the many combinations for the flu strain that contains the HA and NA genes from flu strain 1 and remaining genes from flu strain 2 that ensures that it is able to grow efficiently in eggs.
• This new reassortant flu strain and two other flu strains will make up next year’s vaccine.

source:National Institute of Allergy and Infectious Diseases (NIAID). 
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