Mutations to A/Puerto Rico/8/34 PB1 gene improves seasonal reassortant influenza A virus growth kinetics

► Influenza vaccine strains contain variant PB1 genes. ► Targeted mutagenesis of PR/8 PB1 can improve vaccine strain characteristics. ► Specific PB1 mutations enhance growth of poorly growing virus. ► Specific PB1 mutations increase protein yield in allantoic fluid at early time points.