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).

• 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.

Click the illustration to the right to see it in full size.

More recently an alternative technique to Reassortment has been developed, called Reverse Genetics. This can been used to modify the virus to render it non-pathogenic to eggs – enabling avian strains like H5N1 to be successfully grown in eggs. This technique is also described in the above link and in the Pandemic Production section.