Elecsys anti sars cov 2 n immunoassay

Concentration of IgG antibodies directed to the SARS-CoV-2 spike (S) protein receptor binding domain (RBD) was determined in plasma by Elecsys^® Anti-SARS-CoV-2 S immunoassay (Roche Diagnostics GmbH, Mannheim). SARS-CoV-2 nucleocapsid (N)-specific antibodies (both IgG and IgM) were determined in plasma using an Elecsys^® Anti-SARS-CoV-2 N immunoassay (Roche), and categorized as seropositive and seronegative based on the antibody cut-off index (COI ≥ 1.0, reactive; COI < 1.0, non-reactive). Participants positive for N-specific antibodies in mRNA and adenovector vaccine groups were considered as individuals previously exposed to or infected with SARS-CoV-2. With inactivated whole-virus vaccine, it is not possible to differentiate whether N-specific antibody was generated due to vaccination or natural infection with SARS-CoV-2.

For antibody measurement, venous blood samples were collected before booster vaccination as well as 3 days and 3 weeks after the booster. For a subset of n = 19 participants, concentrations were also measured 1 and 5 weeks after booster vaccination. Anti-S antibody concentrations were determined using the Elecsys Anti-SARS-CoV-2 S immunoassay (Roche, Basel, Switzerland). The test detects IgG, IgM, and IgA (quantitative), and has a sensitivity of 100% (4 weeks after positive PCR) as well as a specificity of 99.98%. The assay used a cut-off point of 0.8 U/mL to classify samples as reactive (≥0.8 U/mL) or non-reactive (<0.8 U/mL). Anti-S antibody concentrations are reported in U/mL. According to the manufacturer, 1 U/mL is equivalent to 1 BAU/mL (WHO standard). Antibodies against the N-Protein were measured with the Elecsys Anti-SARS-CoV-2 N immunoassay (Roche, Basel, Switzerland), which has a sensitivity of 99.5% (14 days after positive PCR), and a specificity of 99.5%. The test detects IgG, IgM, and IgA (qualitative).

For this study, we used the quantitative Elecsys anti-SARS-CoV-2 RBD immunoassay, which measures total antibodies (IgG/A/M) against the receptor binding domain of the virus spike (S) protein (#09 289 275 190, Roche-S, Roche Diagnostics, Rotkreuz, Switzerland). Seropositivity was defined using the cut-off provided by the manufacturer of ≥0.8 U/mL. Output test values were transformed to WHO international standard units by multiplying by a factor of 1.184. We calculated the intra-lot coefficient of variation (CV) for each batch of our internal positive control serum, and the maximum CV (7.3%) was used to define uncertainty in serological measurements in the kinetic model described below (Supplementary Material Section S2). To identify past infections in vaccinated participants, we also measured total levels of antibodies binding the nucleocapsid (N) protein using the semiquantitative Elecsys anti-SARS-CoV-2 N immunoassay (#09 203 079 190, Roche-N). The three vaccines available in Switzerland during the study period elicit a response exclusively to the Spike protein of SARS-CoV-2, as opposed to infection, typically eliciting a response to both the N and S virus proteins. Although not the focus of the main analysis, we also present anti-N antibody trajectories in the Supplementary Material (Supplementary Material Section S3).