<Dl> <Dd> "The WHO Global Influenza Surveillance Network was established in 1952 . The network comprises four WHO Collaborating Centres (WHO CCs) and 112 institutions in 83 countries, which are recognized by WHO as WHO National Influenza Centres (NICs). These NICs collect specimens in their country, perform primary virus isolation and preliminary antigenic characterization . They ship newly isolated strains to WHO CCs for high level antigenic and genetic analysis, the result of which forms the basis for WHO recommendations on the composition of influenza vaccine for the Northern and Southern Hemisphere each year ." </Dd> </Dl> <Dd> "The WHO Global Influenza Surveillance Network was established in 1952 . The network comprises four WHO Collaborating Centres (WHO CCs) and 112 institutions in 83 countries, which are recognized by WHO as WHO National Influenza Centres (NICs). These NICs collect specimens in their country, perform primary virus isolation and preliminary antigenic characterization . They ship newly isolated strains to WHO CCs for high level antigenic and genetic analysis, the result of which forms the basis for WHO recommendations on the composition of influenza vaccine for the Northern and Southern Hemisphere each year ." </Dd> <P> The Global Influenza Surveillance Network's selection of viruses for the vaccine manufacturing process is based on its best estimate of which strains will predominate the next year, amounting in the end to well - informed but fallible guesswork . </P> <P> Formal WHO recommendations were first issued in 1973 . Beginning in 1999 there have been two recommendations per year: one for the northern hemisphere (N) and the other for the southern hemisphere (S). </P>

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