ABSTRACT
Background. Poor immunogenicity and antibody waning were found in vaccinees of CoronaVac. There is lack of randomized controlled trial (RCT) data to compare the immunogenicity and safety of schedules using homologous and heterologous vaccine as a booster dose. Methods. We randomly assigned adults who had received 2 doses of CoronaVac with low antibody response to receive an additional booster dose of either BNT162b2 or CoronaVac. The local and systemic adverse reactions were recorded. Levels of SARS-CoV-2 neutralizing and spike binding antibody in plasma were measured. Findings. At one month after the third dose of vaccine, BNT162b2 vaccines elicited significantly higher surrogate virus neutralizing test (sVNT), spike receptor binding, spike N terminal domain binding, spike S2 domain binding levels than CoronaVac. More participants from the BNT162b2 group reported injection site pain and swelling as well as fatigue and muscle pain than those who received CoronaVac as the third dose. The mean results of the sVNT against the wild type, beta, gamma and delta variants in the BNT162b2 boosted group was 96.83%, 92.29%, 92.51% and 95.33% respectively which were significantly higher than the CoronaVac boosted group (Wild type: 57.75%; Beta: 38.79 %; Gamma: 32.22%; Delta: 48.87%) Conclusion. Our RCT study shows that BNT162b2 booster dose for those people who poorly responded to the previous vaccination of CoronaVac is significantly more immunogenic than a CoronaVac booster. BNT162b2 also elicits higher levels of SARS-CoV-2 specific neutralizing antibodies to different variants of concern. The adverse reactions were only mild and short-lived.