The comparison of Immune response to Third booster dose with of ChAdOx-1 or BNT162b2 COVID-19 vaccine in healthcare workers after receiving two doses of CoronaVac

Background The COVID-19 outbreak has significantly impacted public health, with vaccine effectiveness being crucial for control. The efficacy of heterologous prime-boost vaccines remains unclear. This study compared AZD1222 and BNT162b2 as booster doses given 90 days after two CoronaVac doses. Methods Participants who received two CoronaVac doses at least 90 days earlier chose either the AZD1222 or BNT162b2 booster based on individual preference. Anti-Spike receptor-binding domain (anti-S-RBD) IgG levels were measured on days 14, 28, and 90, and surrogate virus neutralization tests (sVNT) were conducted on days 14 and 90. Results A total of 100 subjects participated in the study: 36 males and 64 females. The mean age in the AZD1222 group was 42.82±6.01 years, and in the BNT162b2 group, it was 37.3±8.07 years (p-value <0.05). The mean interval between the previous vaccine dose and the booster dose was 87.3±20.1 days in the AZD1222 group and 109.2±17.6 days in the BNT162b2 group (p-value <0.05). Anti-S-RBD levels in the BNT162b2 group were significantly higher than those in the AZD1222 group on days 14, 28, and 90. The sVNT inhibition of the Delta variant spike protein was also significantly higher in the BNT162b2 group on both days 14 and 90. Age was the only factor affecting anti-S-RBD levels in both groups. Conclusion In participants previously given two CoronaVac doses, both the mRNA BNT162b2 and viral-vectored AZD1222 boosters showed increased immunogenicity and neutralizing activity against the Delta strain. However, BNT162b2 demonstrated slightly higher immunogenicity and a slower decline over a 3-month period compared to AZD1222.