Antibody and antigen testing
Serum IgG antibodies against S1 and NCP antigens of SARS-CoV-2 were measured by commercial ELISAs (Anti-SARS-CoV-2-ELISA [IgG] and Anti-SARS-CoV-2-NCP-ELISA [IgG]; Euroimmun, Lübeck, Germany), according to the manufacturer’s instructions. Upon manual dilution, sera were processed in a BEPIII analyzer (Siemens, Eschborn, Germany). Results are indicated as Ratio (cutoff: <0.8; borderline: ≥0.8, <1.1; positive: >1.1). Circulating NCP antigen in serum was quantified using the ScheBo® SARS-CoV-2TM Antigen ELISA (ScheBo Biotech, Gießen, Germany). Values ≥2.97 pg/mL were considered positive.