Immunogenic Properties and Safety of a Quadrivalent Inactivated Subunit Adjuvanted Influenza Vaccine in Adults Aged 18 to 85 Years at the End of the COVID-19 Pandemic in the 2022–2023 Season

<b>Background</b>: SARS-CoV-2 infection has raised concerns about altered immune responses, creating a need to evaluate influenza vaccine performance in the post-COVID period. This study aimed to compare the immunogenicity and safety of a quadrivalent inactivated subunit adjuvanted influenza vaccine in adults aged 18-85 years during the 2022-2023 season. <b>Methods</b>: A total of 144 adults were enrolled: group 1, aged 18-59 years (<i>n</i> = 124), and group 2, aged 60-85 years (<i>n</i> = 20). All received a quadrivalent inactivated subunit adjuvanted vaccine containing 5 μg of each influenza antigen and 500 μg of Azoximer bromide. IgG antibodies to vaccine strains were measured at baseline and days 30-32 using the hemagglutination inhibition assay. Participants were actively monitored for adverse events by telephone. <b>Results</b>: The Geometric Mean Fold Increase (GMFI) met the efficacy criteria in both age groups (≥2.5 for 18-59 years and ≥2.0 for 60-85 years), with no significant differences. The seroprotection rate reached accepted thresholds for most strains but was below criteria for B/Victoria in the 18-59 group (48%) and for B/Phuket in the 60-85 group (35%). Significant between-group differences were observed for B/Victoria (<i>p</i> = 0.01) and B/Phuket (<i>p</i> = 0.007). Seroconversion met criteria for all strains in younger adults, but for older adults, it was insufficient for B/Phuket (20%, below the ≥30% threshold; <i>p</i> = 0.05 vs. 18-59 years). Local reactions occurred in 24.2% and systemic in 11.3% of younger adults; in older adults, in 20% and 15%, respectively. All resolved spontaneously within 1-3 days. <b>Conclusions</b>: The quadrivalent adjuvanted influenza vaccine demonstrated acceptable immunogenicity and safety in adults aged 18-85 years despite potential post-COVID immune alterations.