Presence of SARS-CoV-2-neutralizing antibodies over several months was demonstrated in eight (72.7%) SARS-CoV-2 patients and a family member of patient no. 14 (R4). However, only one of them developed a stable titer >1:40 (Figure 5 and Figure S2). Retests of sera that were already included in the validation part of this study showed an excellent reproducibility of the PRNT (R2 = 0.96, Figure S4).