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." The Global Influenza Surveillance and Response System'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. Formal WHO recommendations were first issued in 1973. Beginning in 1999 there have been two recommendations per year: one for the northern hemisphere and the other for the southern hemisphere. Historical annual reformulations of the influenza vaccine are listed in a separate article. WHO seasonal influenza vaccine composition recommendations: 2020–2021 northern hemisphere The composition of virus vaccines for use in the 2020–2021 Northern Hemisphere influenza season recommended by the World Health Organization on February 28, 2020, is: for egg-based: an A/Guangdong-Maonan/SWL1536/2019 (H1N1)pdm09-like virus an A/Hong Kong/2671/2019 (H3N2)-like virus a B/Washington/02/2019 (B/Victoria lineage)-like virus a B/Phuket/3073/2013 (B/Yamagata lineage)-like virus for cell- or recombinant-based: an A/Hawaii/70/2019 (H1N1)pdm09-like virus an A/Hong Kong/45/2019 (H3N2)-like virus a B/Washington/02/2019 (B/Victoria lineage)-like virus a B/Phuket/3073/2013 (B/Yamagata lineage)-like virus The WHO recommends that trivalent vaccines use as their influenza B virus a B/Washington/02/2019 (B/Victoria lineage)-like virus. United States The Vaccines and Related Biological Products Advisory Committee (VRBPAC) of the Food and Drug Administration (FDA) recommended that the quadrivalent formulation of egg-based influenza vaccines for the US 2020–2021 influenza season contain the following: an A/Guangdong-Maonan/SWL1536/2019 (H1N1)pdm09-like virus; an A/HongKong/2671/2019 (H3N2)-like virus; a B/Washington/02/2019-like virus (B/Victoria lineage); a B/Phuket/3073/2013-like virus (B/Yamagata lineage). The committee recommended that the quadrivalent formulation of cell- or recombinant-based influenza vaccines for the US 2020–2021 influenza season contain the following: an A/Hawaii/70/2019 (H1N1)pdm09-like virus; an A/HongKong/45/2019 (H3N2)-like virus; a B/Washington/02/2019-like virus (B/Victoria lineage); a B/Phuket/3073/2013-like virus (B/Yamagata lineage) For trivalent influenza vaccines for use in the US for the 2020–2021 influenza season, depending on the manufacturing method of the vaccine, the committee recommended that the A(H1N1)pdm09, A(H3N2) and B/Victoria lineage viruses recommended above for the quadrivalent vaccines be used. European Union The composition of virus vaccines for use in the European Union for the 2020–2021 Northern Hemisphere influenza season recommended by the