Comparison of Anti-receptor Binding Domain of SARS-CoV-2 Spike Protein Humoral Response between Infected, Vaccinated and Vaccinated after Natural Exposure Subjects over 12-months Follow-up Period

The duration and dynamics of the humoral immune response against SARS-CoV-2 in different groups of individuals are important for determining the risk of reinfections and breakthrough infections. The aim of this study was to monitor and compare the humoral immune response against SARS-CoV-2 over a 12-month period by testing anti-RBD IgG, anti-S IgA, and virus neutralizing antibodies (VNAb). The study included 107 participants belonging to the following groups: COVID-19 convalescent individuals (CI), vaccinated seronaive (VS) and vaccinated convalescent participants (VC). The mean values of anti-RBD IgG and VNAb antibodies in period I were statistically significantly higher in the two groups of vaccinated participants compared to CI. In period II and period III, the mean IgG and IgA levels were statistically significantly higher in VC individuals compared to CI or VS. The mean values of anti-S IgA in all three periods were statistically higher in the group of participants with hybrid humoral immunity compared with CI and VS. The results showed that after the third month and until the end of the study, the mean anti-RBD IgG, anti-S IgA, and VNAb antibody values remained highest in the vaccinated convalescents. In vaccinated seronaive the mean values for the three parameters in the first three months were higher than those in CI. A greater decline was observed in the group of VS and after 7 months the mean anti-RBD IgG, anti-S IgA, and VNAb values in this group were similar to those of the convalescents-only group.