Flu Vaccine

The graphic shows an epidemic viral strain (purple virion) being introduced into embryonated (i.e. fertilised) chicken eggs. The green virion represents a standard strain that grows easily in chicken eggs (A/Puerto Rico/8/34 - A/PR8). These two types replicate alongside each other in an individual egg. As they copy themselves, their genes get jumbled together. This happens because influenza stores its genetic material in eight separate segments and a progeny virus can source segments from different parent viruses. The resulting hybrid virus strains (also called reassortants) are vetted and one that contains genes for the purple surface proteins but all the other genes from the green strain is chosen as the new vaccine strain. This vaccine strain (translucent purplish virions in the yellow cone) is then grown in millions of eggs.