A Longitudinal Study of Humoral Immune Responses Induced by a 3-Dose Inactivated COVID-19 Vaccine in an Observational, Prospective Cohort

Objective: To quantify the dynamic SARS-CoV-2 specific antibody levels induced by an inactivated COVID-19 vaccine, CoronaVac.Methods: An observational, prospective cohort study was performed with 93 healthy healthcare workers from a tertiary hospital in Nanjing, China. Serum SARS-CoV-2 specific IgM, IgG, and ACE2-blocking antibodies were measured at seven time points among participants who received 3 doses of inactivated COVID-19 vaccine.Results: SARS-CoV-2 specific IgG and ACE2-blocking antibody seroconversion rate is high after receiving primary and booster immunization. Besides, vaccine immune responses were negatively correlated with age. Additionally, SARS-CoV-2 specific IgG and ACE-2-blocking antibody titers were highly correlated with neutralization titers of D614G, Omicron, and Delta strain.Conclusions: CoronaVac elicited robust SARS-CoV-2 specific IgM, IgG and ACE2-blocking antibody responses. Monitoring SARS-CoV-2 antibody responses induced by vaccination would provide important guidance for the immunogenicity evaluation of COVID-19 vaccines.