To determine if this enhanced high-yield vaccine virus backbone also increased the titers of other vaccine viruses, we combined the S12-HY vaccine backbone with the HA and NA vRNAs of three other human influenza vaccine strains: A/Switzerland/8060/2017 (H3N2; Switzerland; clade 3C.2a2), A/Kansas/14/2017 (H3N2; Kansas; clade 3C.3a1), and A/Michigan/45/2015 (H1N1; Michigan; clade 6B.1) (Table S2). This evidence concerns the gene XK and influenza.