The nanoemulsion adjuvant provides antigen dose-sparing effects and enhances maternal passive immune protection for the cell-cultured quadrivalent influenza virus subunit vaccine.

To elicit a robust immune response, an adjuvant can be combined with the antigen in influenza vaccine formulations. In this study, we evaluated the dose-sparing effect and safety of a squalene-based oil-in-water nanoemulsion (NE) adjuvant formulated with a cell culture-derived quadrivalent influenza vaccine. Immune responses-including anti-HA IgG antibody levels and hemagglutination inhibition (HAI) titers-were assessed, along with protection against a homologous challenge with influenza B virus (strain B/Maryland/15/2016 B/Victoria). We also investigated the influence of antigen dose on vaccine-induced immunity and the passive protection conferred to offspring via maternal antibody transfer. The NE adjuvant elicited strong anti-HA antibody responses in young adult mice, and these antibodies were effectively transferred from immunized mothers to their offspring. Furthermore, offspring born to NE-immunized mothers were protected against influenza virus challenge. Collectively, our results indicate that the NE formulation induces potent influenza-specific immune responses with dose-sparing effects and enables maternal transfer of protective immunity. These findings support the potential of NE as an effective adjuvant for MDCK cell-based influenza subunit vaccines. Importance. This study demonstrates that a squalene-based nanoemulsion (NE) adjuvant significantly enhances the immunogenicity and dose-sparing capacity of cell culture-derived quadrivalent influenza vaccines. Key advances include: (1) NE-adjuvanted vaccines achieved a remarkable 125-fold antigen dose reduction while maintaining antibody titers comparable to high-dose formulations, addressing critical challenges in pandemic preparedness; (2) NE induced robust humoral and cellular immunity, including elevated anti-HA IgG (10-fold increase vs. non-adjuvanted vaccine), improved HI titers, and enhanced IFN-γ/CD8 + T-cell responses; (3) Unique maternal-offspring protection was demonstrated, with transferred maternal antibodies conferring 100% survival in offspring against viral challenge. These findings position NE as an effective adjuvant technology that simultaneously optimizes antigen use and broadens immune protection across age groups.