All collected samples in March, May, and November 2021 were tested using the Roche Elecsys anti-spike protein of SARS-CoV-2 S, measuring total antibodies [6 –8 (link)]. The Roche assay is an electrochemiluminescent immunoassay (ECLIA) that quantitatively detects antibodies to the SARS-CoV-2 spike protein receptor binding domain (including IgM and IgG) (Roche Diagnostics K.K., Japan). The assay results were interpreted using the manufacturers’ recommended specific thresholds. This assay is highly sensitive and specific [7 (link)]. Assays with good sensitivity have a lower percentage of false negatives. This assay has a measuring range of 0.40–250 U/mL (up to 2500 U/mL with on-board 1:10 dilution), with a concentration of < 0.80 U/mL considered negative and ≥ 0.80 U/mL considered positive.
To compare with an antibody test against spike protein of SARS-CoV-2, we also measured 636 serum samples collected in November 2021 with an anti-nucleocapsid protein of SARS-CoV-2 using an ECLIA kit obtained from Roche diagnostics K.K. (Japan). [10 –14 (link)] According to the manufacturer’s instructions, the cut-off value for a positive SARS-CoV-2 antibody for nucleocapsid protein was deemed as 1 COI (cut-off index).
To compare with an antibody test against spike protein of SARS-CoV-2, we also measured 636 serum samples collected in November 2021 with an anti-nucleocapsid protein of SARS-CoV-2 using an ECLIA kit obtained from Roche diagnostics K.K. (Japan). [10 –14 (link)] According to the manufacturer’s instructions, the cut-off value for a positive SARS-CoV-2 antibody for nucleocapsid protein was deemed as 1 COI (cut-off index).
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