Figure 2. Distribution of Spike Receptor-Binding Domain (S-RBD) IgG Titers .

Note the progressive decrease of median antibody titers from children younger than 3 years to adults (age ≥18 years). Lines and whiskers represent medians and IQRs. The dotted line corresponds to the assay cutoff for discriminating positive from negative samples. BAb indicates early-binding antibody; kBAU, kilo–binding antibody units.