Since 1999, the World Health Organization has issued annual recommendations for influenza vaccine formulations. One reformulation of the influenza vaccine is for the Northern Hemisphere, and the other is for the Southern Hemisphere. Both recommendations are trivalent, i.e. featuring three strains.
Since the 2012–13 season, the WHO recommendations have also included the formulation of an annual quadrivalent vaccine, featuring an additional B-strain.
Historical annual reformulations of the influenza vaccine Wikipedia
The following is a list of WHO recommended strains for the Northern Hemisphere influenza season.
The following is a list of WHO recommended strains for the southern hemisphere influenza season.