Clinical Utility of SARS-CoV-2 Antibody Titer Multiplied by Binding Avidity of Receptor-Binding Domain (RBD) in Monitoring Protective Immunity and Clinical Severity

Conventional serum antibody titer, which expresses level, does not provide antigen binding avidity of the variable region antibody, is essential for defense response to infection. Here, we quantified anti-SARS-CoV-2 receptor-binding domain (RBD) by competitive binding-inhibition activity (IC50) between SARS-CoV-2 S1 immobilized on DCP microarray and various RBD doses added expressed as 1/IC50 nM. The analyzed under equilibrium conditions antigen–antibody reaction different from index measured with chaotropic agent, such urea, nonequilibrium short-time conditions. Quantitative determination infection-protection potential antibodies was assessed ABAT (antigen titer), calculated quantity (level) × quality (binding avidity) antibodies. correlated strongly (r = 0.811) cell-based virus-neutralizing activity. Maturation protective induced repeated vaccinations or infection classified into three categories ABAT, an initial, low, high ABAT. Antibody maturity clinical severity COVID-19. Once a mature achieved, it maintained at least 6–8 months regardless subsequent change in levels.