Since 1999, the World Health Organization (WHO) has issued annual recommendations for influenza vaccine formulations.
[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, B/Yamagata.
[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.
[3][4][5] The following is a list of WHO recommended strains for the Northern Hemisphere influenza season.
[2] The following is a list of WHO recommended strains for the Southern Hemisphere influenza season.