Vaccine effectiveness for the pandemic H1N1 component of the FluMist/Fluenz live attenuated influenza vaccine was lower than expected during the 2013–14 and 2015–16 influenza seasons in the USA. Previous studies highlighted the importance of vaccine viruses replicating sufficiently well in human cells that they could compete in quadrivalent formulations and protect ferrets against influenza-like illness. Here, it was demonstrated that mutagenesis of the haemagglutinin surface protein can be used to improve vaccine effectiveness of a pandemic H1N1 vaccine strain. This was achieved by optimising replication in human cells while, crucially, leaving other key characteristics required for vaccine production unaffected. The mutated vaccine virus successfully overcame inter-strain competition, providing protection from influenza-like illness in ferrets, while its parent did not. Furthermore, it was demonstrated that enhanced binding to human-like receptors was the mechanism by which replicative fitness was being improved. These data suggested that improving replicative fitness, allowing the vaccine virus to drive a protective immune response in quadrivalent formulations, would result in increased effectiveness in patients. Subsequently, high vaccine effectiveness was demonstrated during the 2017–18 northern hemisphere influenza season, validating this approach.