The constantly evolving nature of influenza viruses requires continuous global monitoring and frequent reformulation of influenza vaccines.

The World Health Organization (WHO) convenes technical consultations in February and September each year to recommend viruses for inclusion in seasonal influenza vaccines for the northern and southern hemispheres, respectively. These recommendations are based on information provided by the WHO Global Influenza Surveillance Network (GISN), now the WHO Global Influenza Surveillance and Response System. Since 2004, influenza A(H5N1), A(H9N2) and other subtypes of influenza viruses have also been taken into consideration by GISRS for pandemic preparedness purposes.

The development of high yield candidate vaccine viruses is a complex process, involving collaboration of laboratories involved in developing reassortants and WHO Collaborating Centres (CCs). Two technologies are currently being used: classical reassortment (available since 1971) and reverse genetics, a patent technology.

Once developed, these candidate reassortants are sent to WHO CCs for characterization of their antigenic and genetic properties before being released to interested institutions on request. Reference reagents are subsequently developed and standardized by Essential Regulatory Laboratories (ERLs), in collaboration with vaccine manufacturers and made available to manufacturers worldwide upon request.