Historical annual reformulations of the influenza vaccine

Since 1999, the World Health Organization (WHO) 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 2012–2013 season, the WHO recommendations have also included the formulation of an annual quadrivalent vaccine, featuring an additional strain of Influenza B virus, B/Yamagata.

Due to the widespread use of non-pharmaceutical interventions at the beginning of the COVID-19 pandemic, the B/Yamagata influenza lineage has not been isolated since March 2020, and may have been eradicated. Starting with the 2024 Southern Hemisphere influenza season, the WHO and other regulatory bodies have removed this strain from influenza vaccine recommendations.

Northern Hemisphere recommended strains
The following is a list of WHO recommended strains for the Northern Hemisphere influenza season. Starting in the 2012–2013 season, the recommendation shifted to include the composition of a quadrivalent influenza vaccine (QIV) that contains both influenza B lineages, alongside a trivalent influenza vaccine (TIV) containing one influenza B lineage.

Southern Hemisphere recommended strains
The following is a list of WHO recommended strains for the Southern Hemisphere influenza season.