Fig. 4. (A, B) Avidity-maturation of anti-receptor binding domain (RBD) immunoglobulin G (IgG) and anti-S IgG in coronavirus disease (COVID)-naive vaccinees (individuals a, b, c) between day 17–19 and day 51–54 after vaccination (vaccinations were performed at day 1 and 28). Red arrows indicate the cut-off for high-avidity IgG according to the manufacturer’s instructions (>60%).