Evaluation of Quantitative IgG and Interferon-Gamma Responses After Different Booster Strategies of CoronaVac and BNT162b2 Vaccines in Turkey

Abstract Background The global effort to combat the COVID-19 pandemic requires a comprehensive assessment of vaccine efficacy, humoral and cellular immune responses. The current study aimed to determine the effects of CoronaVac and BNT162b2 boosters on quantitative IgG and interferon-gamma (IFN-γ) responses of individuals primed with two doses of CoronaVac in Turkey. This prospective cohort study included 48 participants aged 18–59 years, without any comorbidities and were not under drug therapy, with no clinical history of COVID-19. The study was conducted in three groups: Group 1 was composed of individuals immunized with three doses of CoronaVac; Group 2 two doses of CoronaVac and one dose of BNT162b2; Group 3 two doses of CoronaVac plus two doses of BNT162b2. Humoral immunity was assessed by the determination of the IgG levels against the spike RBD protein of SARS-CoV-2 and cellular immunity by the IFN-γ release assay. Results When the 6–12 month post-vaccination period was considered, the lowest quantitative IgG levels were detected in group 1 in which the booster was applied as CoronaVac. IgG levels were higher in the two groups with BNT162b2 boosters, group 3 (two BNT boosters) revealed the highest levels. The highest IFN-γ response was observed in the group with two BNT162b2 booster applications. Although the difference between the IFN-γ levels was not statistically significant for the three groups, the individuals boosted with the mRNA vaccine revealed two- and three-fold higher levels in comparison to the homologous boosted individuals. No significant gender difference was found for IgG and IFN-γ values in the three groups. The IgG and IFN-γ median values of the younger participants were significantly higher than those of the older participants in Group 3. Conclusions We conclude that although both homologous and heterologous boosting in inactivated vaccine-primed individuals provided effective humoral and cellular immunity, boosting with two doses of BNT162b2 should be prioritized, particularly in people with a higher risk of infection with SARS-CoV-2, since it exhibited a clearly positive impact on both humoral and cellular immunity.