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Anti–SARS-CoV-2 IgG antibodies directed against the S1 domain of the spike protein of the virus using a commercial ELISA kit (Euroimmun, Lübeck, Germany) and quantitative results were expressed in standardised units (binding antibody units [BAU] per mL) as recommended by the manufacturer. The dotted line represents the positive threshold of 35.2 BAU/mL. ELISA, enzyme-linked immunosorbent assay.
