Effect of BNT162b2 and CoronaVac boosters on humoral immunity of individuals previously fully vaccinated with CoronaVac against SARS-CoV-2: A longitudinal study

It is essential to know about the immune response levels after booster doses of the two different types of vaccines, the mRNA and the inactivated, currently used against COVID-19. For this purpose, it was aimed to determine the effects of BNT162b2 (BNT) and CoronaVac (CV) boosters on the humoral immunity of individuals who had two doses of CV vaccination. For the estimation of humoral immune responses, live SARS-CoV-2 virus was isolated from nasopharyngeal swabs of SARS-CoV-2 PCR test positive patient and grown on the Vero E6 cells. After the confirmation of growth and estimation of the TCID50 for the virus, the virus was used in the PRNT50 experiments. For the study the healthcare workers who had no history of Covid-19 infection and were immunized with two doses of inactivated vaccine CoronaVac were included. Blood samples from these individuals were collected at different time points. From whole blood samples, PBMCs and sera were isolated. PBMCs were used for the flow cytometry experiments and sera were used for the neutralizing antibody experiments. In the neutralizing assay, the PRNT50 titer was calculated a dilution in which half of the virus at 10-3 PFU/ml concentration was inactivated and was selected for everyone in all time points. The percentage of B cell subtypes and proportion of memory B cells were estimated for each time point by staining with specific B cell markers (Anti-CD19, Anti-CD20, Anti-IgG, Anti- CD27, Anti- CD14, Anti-CD38 and Anti-CD56). After staining, samples were run by Attune flow cytometry. After the experiments, a 3.38-fold increase in neutralizing antibody geometric mean titers (Neutralizing Antibody, GM, 78.69) was found one month after BNT162b2 booster and maintained at the third month (Neutralizing antibody, GM, 80). Nevertheless, in the CoronaVac booster group, significantly lower neutralizing antibodies, GMT than BNT162b2 after 1 month and 3 months were observed (21.44 and 28.44, respectively) (p<0.001). In the B cell part, significantly higher expression of memory B cell and memory B cells expressing IgG was observed in the BNT162b2 booster group when compared with CoronaVac booster group both in 1st and 3rd month after booster vaccination. The ratio of effector memory B cells 1st month after vaccination in the CoronaVac booster group was significantly higher than BNT162b2 booster group (p=0.0263).