Effective vaccines are essential for controlling the coronavirus disease 2019 (COVID-19) pandemic. CoronaVac, which is an inactivated virus vaccine, was the first imported COVID-19 vaccine in Thailand. To investigate the safety and immunogenicity of CoronaVac within the Thai population, we conducted a prospective cohort study among health care workers aged 18-59 years, who received a 2-dose regimen of CoronaVac 21 days apart between March and April 2021 at the hospital in Samut Sakhon, Thailand. We recruited 185 participants with a mean age of 32 years. Total antibodies against receptor-binding domain (RBD) and immunoglobulin G (IgG) against nucleocapsid (N) protein of SARS-CoV-2 were tested. Total antibodies against RBD were negative before immunization. One volunteer was positive for N, although negative for the RBD antibodies. The seroconversion rate of total antibodies against RBD after the first CoronaVac dose was 67% with a Geometric mean concentration (GMC) of 1.98 U/ml. Following CoronaVac dose 2, the seroconversion rate increased to 100% with a GMC of 92.9 U/ml. The seroconversion rates of IgG against N protein were 1% after dose 1 and 62.8% after dose 2. The overall incidence of adverse reactions was 59.5%. Injection-site pain was the most common local adverse event (52.4%), while myalgia was the most common systemic adverse event (31.9%). No serious adverse events were observed. A 0-21 days, 2-dose CoronaVac regimen appears safe, inducing a satisfactory response compared with convalescent serum obtained 4-6 weeks postnatural infection. Antibody responses after 2-dose CoronaVac were comparable to the convalescent plasma but waned rapidly after 3 months. Therefore, we recommend 2-dose CoronaVac administration with possible booster doses.
Keywords: COVID-19; RBD; immunogenicity; inactivated vaccine; nucleocapsid protein; safety; spike protein.
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