The convalescent sera were also tested with and without avidity reagent (contains urea in denaturating concentrations) in two versions of an IgG line assay (blot v1: Mikrogen recomLine Coronavirus IgG (Avidität), prototype and blot v2: Mikrogen recomLine SARS-CoV-2 IgG (Avidität)/RUO). The first version is based on the N protein of the human coronaviruses (HCoVs) 229E, HKU1, NL63, and OC43, which are used separately as antigens, and on the N protein of the “classic” SARS-CoV from 2002/2003 and SARS-CoV-2. The second improved version still uses the N proteins of the aforementioned HCoVs and of SARS-CoV-2 but has been expanded to include the S1 antigen and its RBD. A Dynablot Plus system was used to process all samples. Blots were then evaluated automatically with a BLOTrix reader and the recomScan software (all from Mikrogen).