AdCLD-CoV19-1, a chimeric adenovirus-based severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccine, was previously reported to elicit robust antibody responses in mice and non-human primates after a single dose. In this study, we conducted a systems serology analysis to investigate changes in humoral immune responses induced by varying doses of the AdCLD-CoV19-1 vaccine in a phase I clinical trial. Serum samples from participants receiving either a low or a high dose of the vaccine were analyzed for antibody features against prototype SARS-CoV-2 spike (S) domains (full-length S, S1, S2, and receptor binding domain), as well as Fc receptor binding and effector functions. While both low- and high-dose vaccines induced robust humoral immune responses following vaccination, the quality of antibody features differed between the dose groups. Notably, while no significant difference was observed between the groups in the induction of most S1-specific antibody features, the high-dose group exhibited higher levels of antibodies and a stronger Fc receptor binding response specific to the S2 antigen. Moreover, univariate and multivariate analyses revealed that the high-dose vaccine induced higher levels of S2-specific antibodies binding to FcγR2A and FcγR3B, closely associated with antibody-dependent neutrophil phagocytosis (ADNP). Further analysis using the Omicron BA.2 variant demonstrated that the high-dose group maintained significantly higher levels of IgG and FcγR3B binding to the S2 antigen and exhibited a significantly higher ADNP response for the S2 antigen compared with the low-dose group. These findings underscore the importance of considering diverse humoral immune responses when evaluating vaccine efficacy and provide insights for optimizing adenovirus vector-based SARS-CoV-2 vaccine doses.IMPORTANCEOptimization of vaccine dose is crucial for eliciting effective immune responses. In addition to neutralizing antibodies, non-neutralizing antibodies that mediate Fc-dependent effector functions play a key role in protection against various infectious diseases, including coronavirus disease 2019. Using a systems serology approach, we demonstrated significant dose-dependent differences in the humoral immune responses induced by the AdCLD-CoV19-1 chimeric adenovirus-based severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccine, particularly against the SARS-CoV-2 spike 2 domain. These findings highlight the importance of assessing not only neutralizing antibody titers but also the quality and functionality of antibody responses when evaluating vaccine efficacy.
Keywords: SARS-CoV-2; adenovirus vector-based vaccine; effector function; spike protein; systems serology.