Copyright © 2017 American Academy of Pediatrics. From: Influenza Red Book® 2015, 2015 Figure Legend: Reassortment. A flu virus contains 8 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 3-2 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 reassorted flu strain and 2 other flu strains will make up next year's vaccine. National Institute of Allergy and Infectious Diseases Date of download: 11/2/2017 Copyright © 2017 American Academy of Pediatrics. All rights reserved.