Serum and Plasma samples from COVID-19 patients were retrieved from EDTA blood samples by centrifugation at 300g for 5 min and serum or plasma fractions were carefully collected and stored at −80°C until use. Patient samples were analyzed by ELISA and ECLIA to assess SARS-CoV-2-specific IgG titers by two commercially available tests; Anti-SARS-CoV-2 ELISA IgG (Euroimmun, Lübeck, Germany) and ElectroChemiLuminescence–Immunoassay (ECLIA) Elecsys® Anti-SARS-CoV-2 (Roche, Basel, Switzerland). SARS-CoV-2 ELISA IgG test detects IgG antibodies against the S protein of SARS-CoV-2. For this test calibrators, controls as well as diluted patient samples were transferred in microplate wells according to pipetting protocol and incubated for 60 min at 37°C. The wells were washed three times using working-strength wash buffer and incubated with peroxidase-labelled anti-human IgG 30 min at 37°C. Wells were washed again and incubated with substrate solution for 30 min at RT followed by addition of stop solution. Photometric measurement was made at 450 nm. For the automated Elecsys® Anti-SARS-CoV-2, which detects IgG antibodies against the N protein of SARS-CoV-2, patient samples were run on a Cobas 8800 System (Roche, Basel, Switzerland) according to the manufacturer’s instructions. Briefly, patient serum or plasma was incubated with a mix of biotinylated and ruthenylated nucleocapsid antigen, which caused double-antigen sandwich immune complexes in the presence of corresponding antibodies. After addition of streptavidin-coated microparticles, the reagent mixture was transferred to the measuring cell and by applying a voltage emitted signals were measured by a photomultiplier. Serum and plasma samples which were tested negative for IgG by ELISA and ECLIA were additionally tested for anti-SARS-CoV-2 IgA and IgM using the commercially available WANTAI SARS-CoV-2 Ab ELISA (Beijing Wantai Biological, Beijing, China). In brief, positive and negative controls as well as specimens are incubated in microwell plate for 30 min at 37°C. Wells are washed five times with wash buffer and wells let 30–60 s to soak. Chromogen solution A and chromogen solution B are mixed 1:1 into each well and incubated for 15 min at 37°C protected from light. After reaction was stopped using stop solution, absorbance was analyzed at 450 nm using a TECAN SUNRICE microplate reader (TECAN, Männedorf, Switzerland).

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