XK and influenza: X-181, which possesses the A/California/07/2009 (H1N1; CA09) HA, NA and PB1 genes and the remaining genes from of the WHO-recommended PR8 virus, did not replicate to high titres, even after an extended period of replication (Fig. 4f); by contrast, the PR8-HY backbone with the CA09 HA and NA genes increased virus titres >250-fold (Fig. 4f, Table 1 and Supplementary Table 10); hence, our high-growth vaccine candidate confers significantly higher virus titres than the NYMC PR8 backbone currently used for the generation of seasonal influenza vaccines.