Since 1999, the World Health Organization (WHO) has issued<unk>  recommendations for influenza<unk>  formulations. One<unk> of the influenza<unk>  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<unk>  quadrivalent<unk> , featuring an additional B-strain. __TOC__
Historical annual reformulations of the influenza vaccine
Global action plan for influenza vaccines
United States influenza statistics by flu season
Vaccine contamination with SV40
Investigations into the origin of COVID-19
Timeline of human vaccines
Vaccine-associated enhanced respiratory disease
HIV/AIDS prophylaxis in British Columbia