Design and performance characteristics of the Elecsys anti-SARS-CoV-2 S assay

Abstract

Background: Automated, high throughput assays are required to quantify the immune response after infection with or vaccination against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). This study on the Roche Elecsys^® Anti-SARS-CoV-2 S (ACOV2S) assay provides insights on the assay design and performance.

Methods: The ACOV2S assay quantifies antibodies to the receptor-binding domain of the SARS-CoV-2 spike protein. The assigned units and the underlying standardization were compared to the international reference standard in BAU/mL. Assay specificity was assessed in samples (n=5981) collected prior to the COVID-19 pandemic and in samples from patients with non-COVID-19 respiratory infections (n=697) or other infectious diseases (n=771). Sensitivity was measured in 1313 samples from patients with mild COVID-19 and 297 samples from patients hospitalized with COVID-19. Comparison of results was performed to a comparator semi-quantitative anti-S1 assay of indirect detection format as well as a commercially available and an in-house version of a surrogate neutralization assay (ACE2-RBD).

Results: The originally assigned units for the ACOV2S assay were shown to be congruent to the units of the First International WHO Standard for anti-SARS-CoV-2 immunoglobulins. Overall specificity was 99.98% with no geographical differences noted and no loss of specificity in samples containing potentially cross-reacting antibodies. High sensitivity was observed, with 98.8% of samples reported to be reactive >14 days after infection and sustained detection of antibodies over time. For all samples, ACOV2S titers and neutralization capacities developed with comparable dynamics. Robust standardization and assay setup enable excellent reproducibility of results, independent of lot or analyzer used.

Conclusion: The results from this study confirmed that ACOV2S is a highly sensitive and specific assay and correlates well with surrogate neutralization assays. The units established for ACOV2S are also interchangeable with the units of the First International WHO Standard for anti-SARS-CoV-2 immunoglobulins. Worldwide availability of the assay and analyzers render ACOV2S a highly practical tool for population-wide assessment and monitoring of the humoral response to SARS-CoV-2 infection or vaccination.

Keywords: COVID-19; SARS-CoV-2; neutralization; quantitative serology; sensitivity; specificity.

Figure 1

Antigen specificity in pre-pandemic samples. Assessment of Spike antigen specificity in pre-pandemic serum samples (n=1047) is shown in (A). Samples with results exceeding the 95% confidence interval of signal distribution in the assessment with Monomeric Spike 1 or RBD, as shown in (A), were resolved in detail in (B). RBD, receptor binding domain.

Figure 2

Correlation of ACOV2S units to the First International WHO Standard for anti-SARS-CoV-2 immunoglobulins. Linearity using a dilution series of the WHO standard for (A) lot DR, (B) lot MP02, and (C) lot MP05.

Figure 3

Sensitivity of the ACOV2S assay. (A) Sensitivity of the ACOV2S assay in samples from (A) all patients, (B) patients with mild symptoms of COVID-19, and (C) patients hospitalized due to COVID-19.

Figure 4

RBD titer dynamics. (A) Spaghetti and smoothened median curves for RBD titers over time for the ACOV2S and comparator assay, and (B) relative recovery at the last sampling timepoint for the ACOV2S and comparator assay (absolute titer values at the last sampling timepoint are plotted in color [right y-axis] and relative recovery is plotted in black [left y-axis]).

Figure 5

Correlation of the ACOV2S assay with results from the cPass neutralization assay. (A) ACOV2S titers and percentage inhibition from the cPass assay from longitudinal samples from exemplary selected individual donors, and (B) comparison of the ACOV2S and cPass results stratified in time buckets after diagnostic PCR and (C) derived qualitative agreements. ACOV2S reactivity in samples with cPass inhibitory capacities is shown as positive percent agreement (PPA), cPass inhibitory capacity ACOV2S reactive samples is shown as positive predictive value (PPV). Analyses were carried out applying both, 0.8 U/mL and 15 U/mL as decision point for relevant ACOV2S reactivity. This analysis used native samples collected from convalescent donors at different timepoints after infection.