An additional oligosaccharide moiety in the HA of a pandemic influenza H1N1 candidate vaccine virus confers increased antigen yield in eggs

► HA substitutions in influenza pandemic H1N1 vaccine were analysed. ► Specific substitutions affecting increased antigen yield identified. ► A specific novel glycosylation site is associated with increased yield. ► Substitutions affecting yield were not transferrable between vaccine viruses.