Each year, three strains are chosen for selection in that year's flu vaccination by the WHO Global Influenza Surveillance and Response System. The chosen strains are the H1N1, H3N2, and Type-B strains thought most likely to cause significant human suffering in the coming season. Starting with the 2012–2013 Northern Hemisphere influenza season (coincident with the approval of quadrivalent influenza vaccines), the WHO has also recommended a 2nd B-strain for use in quadrivalent vaccines. The World Health Organization (WHO) coordinates the contents of the vaccine each year to contain the most likely strains of the virus to attack the next year.