The development of the seasonal influenza vaccine is a complex process that involves global surveillance and identification of the most common circulating strains. The World Health Organization (WHO) collaborates with various national influenza centers around the world to monitor the epidemiology of influenza viruses. Based on the data collected, WHO recommends the strains to be included in the vaccine for the upcoming flu season. This recommendation is made twice a year for the Northern and Southern Hemispheres.
