"The   enables WHO to recommend twice annually the content of the influenza vaccine for the subsequent influenza season (current recommendations.) More than 250 million doses of influenza vaccine are produced annually which contain the WHO recommended influenza strains. Frequent updating of the influenza vaccine content is necessary as influenza viruses are permanently evolving. Only a vaccine whose virus strains match the circulating influenza viruses will protect recipients efficiently from influenza disease and death. The WHO Influenza Surveillance Network serves also as a global alert mechanism for the emergence of influenza viruses with pandemic potential. Its activities have contributed greatly to the understanding of influenza epidemiology. The network was established in 1952, after a WHO Expert Committee recommended that through an international network of laboratories, WHO would be able to advise WHO Member States as to "what influenza control measures are useful, useless or harmful"."