Evaluation of Immunogenicity of Seasonal Influenza Split HA Vaccines Involved With Changes of Vaccine Strains Using Mouse and Human Sera.

Seasonal influenza is prevalent every winter, and influenza vaccines are used to safeguard public health. As the influenza vaccines are produced in embryonated hen eggs using vaccine viruses recommended by the World Health Organization each year based on the epidemic situation, it is important, from the perspective of public health, to evaluate the reactivity of the vaccines against circulating viruses in humans. This study was designed to determine whether evaluating influenza vaccine efficacy using mouse sera could help predict efficacy in humans. The split hemagglutinin (HA) vaccines, produced using vaccine viruses for the 2023/2024 season, were inoculated into ddY and BALB/c mice, and their neutralizing reactivity to circulating influenza viruses was evaluated using their antisera. The titers of antibodies against these viruses in the antisera from humans immunized with the split HA vaccine were also measured. The vaccines induced the production of functional vaccine-specific antibodies dose-dependently. In the case of circulating viruses, the neutralizing antibodies in the sera of immunized individuals were able to react to most the test viruses, but they were less reactive to some viruses. The titers of neutralizing antibodies against these circulating strains in the antisera from humans immunized with the split HA vaccine for the 2023/2024 season were similar to those in the mouse antisera. Furthermore, intraperitoneal inoculation in ddY mice induced antibody production with higher neutralizing titers than subcutaneous inoculation in BALB/c mice. Taken together, the immunization protocol using naive mice could be a suitable method for predicting vaccine efficacy in humans.