Quantifying SARS-CoV-2 Antibody Titers

Anti-spike S1 IgG antibodies were measured using the SARS-CoV-2 Total Assay (Siemens, Eschborn, German). Results of this assay are obtained in as little as 10 minutes on the Atellica IM Analyzer with a capacity to process up to 440 assays per hour, dependent on test mix. Positivity is defined as a semi-quantitative index of ≥1, which gives a specificity of 100% and a sensitivity of 98% for the detection of anti-S1 IgG antibodies. The assay is called semi-quantitative as there is no scale basis for antibody testing for this assay. The clinical applicability of a semi-quantitative assay is currently unknown and cannot be transferred into a degree of immunity. A strong correlation for anti-S1 IgG as measured by the SARS-CoV-2 Total Assay to neutralization against SARS-CoV-2 wild-type or other SARS-CoV-2 variants of concern has been described previously (10 (link), 17 (link)–19 (link)). We further used the Elecsys anti-SARS-CoV-2 assay (Roche, Mannheim, Germany) to detect antibodies against the nucleocapsid protein. Assays were performed according to the manufacturers’ instructions.