Borderline and weakly positive antibody levels against the S-protein of SARS-CoV-2 exhibit limited agreement with virus neutralization titres

Utility of Roche Elecsys anti-SARS-CoV-2 S in ascertaining post-vaccine neutralizing antibodies

With widespread global COVID-19 vaccine coverage, a scalable, cost-effective, and standardized tool to ascertain post-vaccine immunity is a dire need. Neither clinical evaluations of vaccine efficacy, nor live virus antibody neutralization assays fulfill these criteria. Commercially available anti-S binding immunological assays have the potential to fill this gap, but need to be systematically evaluated for their utility to serve as surrogates for the aforementioned, widely accepted tools of determining vaccine efficacy. In this study, we evaluated an anti-S binding immunological assay (Roche Elecsys Anti-SARS-CoV-2 S) by utilizing two hundred and fifty-five archived serum specimens, either pre-pandemic, or those exposed to natural infections or vaccines with their neutralizing titers pre-determined through a live virus, pseudotyped antibody neutralization assay. Roche Elecsys Anti-SARS-CoV-2 S demonstrated good sensitivity (98%) and specificity (99%), just as has been reported in some other previously conducted studies using this assay. Only a mild correlation, however, with the live virus pseudotyped lentivirus antibody neutralization assay (Spearman's r = 0.26) was observed. We conclude that, as such, Elecsys Anti-SARS-CoV-2 S has a high sensitivity and specificity for detecting anti-SARS-CoV-2 S proteins, though the assay does not always correlate well with live virus assays for quantitative outcomes.

One-Year Follow-Up of COVID-19 Patients Indicates Substantial Assay-Dependent Differences in the Kinetics of SARS-CoV-2 Antibodies

Determination of antibody levels against the nucleocapsid (N) and spike (S) proteins of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) are used to estimate the humoral immune response after SARS-CoV-2 infection or vaccination. Differences in the design and specification of antibody assays challenge the interpretation of test results, and comparative studies are often limited to single time points per patient. We determined the longitudinal kinetics of antibody levels of 145 unvaccinated coronavirus disease 2019 (COVID-19) patients at four visits over 1 year upon convalescence using 8 commercial SARS-CoV-2 antibody assays (from Abbott, DiaSorin, Roche, Siemens, and Technoclone), as well as a virus neutralization test (VNT). A linear regression model was used to investigate whether antibody results obtained in the first 6 months after disease onset could predict the VNT results at 12 months. Spike protein-specific antibody tests showed good correlation to the VNT at individual time points (rS, 0.74 to 0.92). While longitudinal assay comparison with the Roche Elecsys anti-SARS-CoV-2 S test showed almost constant antibody concentrations over 12 months, the VNT and all other tests indicated a decline in serum antibody levels (median decrease to 14% to 36% of baseline). The antibody level at 3 months was the best predictor of the VNT results at 12 months after disease onset. The current standardization to a WHO calibrator for normalization to binding antibody units (BAU) is not sufficient for the harmonization of SARS-CoV-2 antibody tests. Assay-specific differences in absolute values and trends over time need to be considered when interpreting the course of antibody levels in patients. IMPORTANCE Determination of antibodies against SARS-CoV-2 will play an important role in detecting a sufficient immune response. Although all the manufacturers expressed antibody levels in binding antibody units per milliliter, thus suggesting comparable results, we found discrepant behavior between the eight investigated assays when we followed the antibody levels in a cohort of 145 convalescent patients over 1 year. While one assay yielded constant antibody levels, the others showed decreasing antibody levels to a varying extent. Therefore, the comparability of the assays must be improved regarding the long-term kinetics of antibody levels. This is a prerequisite for establishing reliable antibody level cutoffs for sufficient individual protection against SARS-CoV-2.

Rapid Quantification of SARS-CoV-2 Neutralising Antibodies Using Time-Resolved Fluorescence Immunoassay

The quantification of neutralising antibodies (NAb) for SARS-CoV-2 has become an important tool for monitoring protective immunity following infection or immunisation. In this study, we evaluated using World-Health-Organisation-standard immunoglobulin preparations, a novel point-of-care test that quantitates NAb by time-resolved fluorescent immunoassay. The assay provided robust data of binding antibody units (BAU) in 15 min that were well correlated with NAb values obtained by traditional in vitro neutralisation assay. The data also correlated well to spike-receptor-binding domain-binding antibodies over a broad range of plasma dilutions. The assay was extremely sensitive, able to detect positive samples after dilution 1:10,000 and over a wide range of BAU. Assay specificity was estimated at 96% using Pre-COVID-19 serum samples when applying a cut-off value of 47 BAU/mL, although readings of up to 100 BAU/mL could be considered borderline. This point-of-care diagnostic test is useful for rapid population screening and includes the use of capillary blood samples. Furthermore, it provides results for SARS-CoV-2 NAb in 15 min, which can inform immediate decisions regarding protective immunity levels and the need for continued COVID immunisations.

Comparison of SARS-CoV-2 spike antibody quantitative titer reporting using the World Health Organization International Standard Units by four commercial assays

The accurate measurement of serological response to SARS-CoV-2 vaccination is needed to correlate responses with effective protective immunity. The World Health Organization (WHO) has created an international standard to allow harmonization of immune response assessment to an arbitrary unit across different commercial assays; however, the accuracy of reporting of SARS-CoV-2 spike antibody titers in international standard units (BAU or IU/mL) from commercial assays is not well studied. Here, we report the performance comparison of four quantitative commercial assays testing for SARS-CoV-2 spike immunoglobins using the WHO's international standard. Sera, EDTA-plasma and heparinized plasma collected from individuals who are vaccine naïve or received BNT162b2 (Pfizer/BioNTech), mRNA-1273 (Moderna) or ChAdOx1-S (Oxford-AstraZeneca) were tested using Abbott Architect AdviseDx SARS-CoV-2 IgG II, DiaSorin LIAISON SARS-CoV-2 TrimericS IgG, Roche Elecsys Anti-SARS-CoV-2 S and GenScript cPass SARS-CoV-2 surrogate virus neutralization assays. The sensitivities ranged from 90% to 100%, and specificities from 88% to 100%. These four assays had excellent agreement (0.79–0.93) and correlation (0.87–0.97); however, Passing-Bablok regression analysis indicated that data generated by these assays were not comparable. Our data suggests that natural SARS-CoV-2 infection elicited a greater antibody response compared to vaccines, evident by a significantly higher neutralizing antibody titer in unvaccinated individuals who seroconverted.