pH stability of the recombinant virus. The recombinant viruses were incubated in each buffer at 37 °C for 10 min, and the viral titers were determined by hemagglutination assay. The results are presented as log2 HA titers at the indicated pH conditions. The error bars denote SD of the mean of three independent titers at each time point. The statistical significance of the differences in pH stability between the wild-type virus and mutant viruses was assessed through the Mann–Whitney test (*p < 0.05).