The objective of our study was to evaluate the antibody responses of health care workers (HCWs) who were vaccinated with booster dose BNT162b2 6 months after 2 doses of the CoronoVac vaccine. The study included 318 HCWs vaccinated with inactive CoronaVac SARS-CoV-2 vaccine in 2 doses. Anti-spike/RBD IgG levels were measured immediately before and 1 month after the booster dose. In the sixth month after Coro-naVac vaccination, the median of antibody levels of 1212.02 AU/ML, while it was 9283 AU/mL after BNT162b2 vaccination. IgG antibody titers of over 1050 AU/mL (which is equivalent to 1:80 dilution in the plaque reduction neutralization test) were detected in HCWs 15.09% and 97.8%, respectively. Our results showed that antibody titers increased 8-fold after the booster dose. We believe that the administration of the mRNA vaccine as a booster dose can provide more effective protection against COVID-19 infection, espe-cially in individuals with risk factors. (c) 2022 Elsevier Inc. All rights reserved.