2.4. Detection of Virus-Specific Antibodies
VIRUS
Automated ECLIA tests were performed with two types of SARS-CoV-2 antibody kits using the Cobas 8000 e801 module (Roche Diagnostics). The Elecsys Anti-SARS-CoV-2 assay (Roche Diagnostics) uses a recombinant protein representing the nucleocapsid (NC) antigen for the qualitative detection of antibodies against SARS-CoV-2. Results with <1.0 cut-off indexes (COI) were interpreted as negative for anti-NC antibodies, and those with ≥1.0 were interpreted as positive for anti-NC antibodies. The Elecsys Anti-SARS-CoV-2 S assay (Roche Diagnostics) uses a recombinant protein representing the spike protein receptor-binding domain to quantitatively determine antibodies against SARS-CoV-2. Results with <0.80 U/mL were interpreted as negative for anti-S-Ab, and those ≥0.80 U/mL were interpreted as positive for anti-S-Ab. The surrogated virus neutralization tests (sVNT) were performed with a cPass SARS-CoV-2 Neutralization Antibody Detection kit (GenScript, Piscataway, NJ, USA) using the SpectraMax 190 (Molecular Devices, San Jose, CA, USA) based on ELISA to detect anti-N-Ab against SARS-CoV-2. The results were interpreted as percentage inhibition (%inhibition) based on OD450 intensity. The manufacturer-recommended cut-off of ≥30% signal reduction was used to indicate the presence of anti-N-Ab. All percent inhibition results were converted to IU/mL of the WHO International Standard using an Excel-based conversion tool [