*2.2. Serum IgA, IgG, and IgM Elisa*

Semi-quantitative SARS-CoV-2 IgA, IgG, and IgM enzyme-linked immunosorbent assay (ELISA) targeting the S1-Domain of the S-spike protein subunit were performed (Euroimmun AG, Lübeck, Germany) according to the manufacturer's instructions. Optical density was determined at a wavelength of 450 nm (OD450) and correction set to 620 nm. Ratios were calculated by Ratio = (Extinction control or sample)/(Extinction calibrator). A calibrator and positive control were provided with each ELISA kit. According to the manufacturer, a ratio of ≥1.1 should be regarded as positive and the manufacturer reports a specificity of 92.5 % for IgA, 99.3 % for IgG, and 98.6 % for IgM.
