Historical annual reformulations of the influenza vaccine
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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.[1] Both recommendations are trivalent, i.e. featuring three strains.[2]
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.[2]
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 has recommended removing B/Yamagata from all influenza vaccines.[3][4] For the 2024–2025 Northern Hemisphere influenza season, the US Food and Drug Administration (FDA) recommends removing B/Yamagata from all influenza vaccines.[5] The European Medicines Agency (EMA) recommends removing B/Yamagata from influenza vaccines for the 2024–2025 seasonal flu vaccine composition.[6]