Antibody response after a third dose mRNA-1273 vaccine among vaccinated healthcare workers with two doses of inactivated SARS-CoV-2 vaccine

BACKGROUND: Health care workers (HCWs), a high-risk group for contracting COVID-19 disease, are being prioritized to receive COVID-19 vaccination. A third dose messenger RNA (mRNA) vaccine, mRNA-1273 (Moderna), after 2 doses of inactivated vaccine (CoronaVac), has been used to increase the level of protection against SARS-CoV-2 among Indonesian HCWs. However, data regarding antibody response after mRNA-1273 booster dose are limited. OBJECTIVE: To evaluate the receptor-binding domain (RBD) of the SARS-CoV-2 spike (S) protein (anti-S) titers induced by the third mRNA-1273 vaccine among fully vaccinated HCWs with CoronaVac. RESULTS: A total of 90 HCWs with no history of SARS-CoV-2 infection and who had received the third dose of vaccination were included in this study. The mRNA-1273 vaccine booster was administered 6 months after completing primary vaccination with CoronaVac. After the third dose, the anti-S antibodies level significantly increased, from a median of 41.7 U/mL (interquartile range [IQR], 22.4-92.5) to 28 394 U/mL (IQR, 20 837-41 646) (p <0.0001). After the third dose, seropositivity with the anti-S antibodies level >210 U/mL was observed in all HCWs. Age was negatively associated with the anti-S antibodies level after the mRNA-1273 booster. CONCLUSION: The heterologous prime booster with CoronaVac and mRNA-1273 vaccine booster elicit a pronounced antibody response against SARS-CoV-2 infection.