Abstract

Annual vaccination is an effective strategy for preventing severe disease caused by seasonal influenza. Quadrivalent influenza vaccines (QIVs) protect against two strains of influenza A and two strains of influenza B, thereby enhancing the host antiviral neutralizing antibody response and inducing CD4(+) and CD8(+) T cell responses. Here, we report findings from a randomized, double-blind, active-controlled phase 3 clinical trial (NCT05431725) that includes 334 healthy adults aged 18-64 years and evaluates the humoral and cellular antiviral immune responses induced by two inactivated QIVs, Sinovac-QIV and Vaxigrip-Tetra (TM). The primary endpoint of the study is the specific antibody responses measured by hemagglutination inhibition (HAI) assays 28 days post-vaccination, while the secondary endpoint is virus-specific T cell responses. Both QIVs elicit significant increases in antibody titers 28 days after vaccination; Sinovac-QIV induces 9-10-fold increases in geometric mean titers, while Vaxigrip-Tetra (TM) elicits 7-8-fold increases (p < 0.05). Cellular immune responses using ELISPOT and supervised and unsupervised flow cytometry analyses show that both vaccines modulate the frequency of hemagglutinin-specific CD4(+) and CD8(+) T cell subsets, inducing distinct T cell response profiles. Although cellular analyses are evaluated in a subgroup of the cohort, the data indicate that both QIVs induce robust humoral and cellular immunity in adults, providing mechanistic insights into vaccine-induced protection.