In the validation part of our study, the Abbott assay, which is based on the SARS-CoV-2 N as the epitope, and the Virion-Serion assay, which uses S as the antigen, proved to be the most sensitive and specific assays (Table 1). Therefore, both were selected to study the kinetics of SARS-CoV-2 IgG in eleven patients and five individuals (including four family members of patient no. 14) from routine diagnostics. The median of the last sample submission was 153 days after the PCR, the range between 80 and 165 days. Results from the validation part of this study were also considered. N-specific IgG antibodies were detectable in all persons. Two groups of individuals were evident: one group of six patients showed relatively stable IgG levels over time, while the other showed a short-term decrease. Accordingly, the indices of six individuals fell below the limit of positivity during the observation period. A stable S-specific IgG response was demonstrated in almost all individuals (Figure 4). Only two subjects (patient no. 22 and routine no. 5) showed a drop in IgG below the limit of positivity. The routine sample no. 5 still possessed isolated reactivity against the N antigen in the improved line assay (data not shown). In the latter test, three patients developed SARS-CoV-2-specific IgG of high avidity after 65–130 days, which was confirmed in consecutive samples. The pattern of reactivity differed slightly: Two patients developed IgG of high avidity against the RBD and S1 epitopes, while the third showed isolated reactivity against the N antigen (Figure S3).