XK and influenza: Of note, our panel of H1N1 viruses includes genetically similar viruses to all World Health Organization (WHO)-recommended H1N1 vaccine strains between 1977 and 2015 (fludb.org/brc/vaccineRecommend.spg?decorator=influenza), antigenically representative viruses circulating in humans in the 1940s31, or representatives of the major HA and NA lineages of seasonal H1N1 viruses30.