Reduced sensitivity of commercial Spike-specific antibody assays after primary infection with the SARS-CoV-2 Omicron variant (preprint)

The SARS-CoV-2 Omicron variant is characterized by substantial changes in the antigenic structure of the Spike (S) protein. Therefore, antibodies induced by primary Omicron infection lack neutralizing activity against earlier variants. In this study, we analyzed whether these antigenic changes impact the sensitivity of commercial anti-SARS-CoV-2 antibody assays. Sera from 37 unvaccinated, convalescent individuals after putative primary Omicron infection were tested with a panel of 20 commercial anti-SARS-CoV-2 immunoassays. As controls, we used samples from 43 individuals after primary infection with the SARS-CoV-2 ancestral wildtype strain. In addition, variant-specific live-virus neutralization assays were used as a reference for the presence of SARS-CoV-2-specific antibodies in the samples. Notably, in Omicron convalescents, there was a statistically significant reduction in the sensitivity of all antibody assays containing S or its receptor-binding-domain (RBD) as antigens. Furthermore, antibody levels quantified by these assays displayed a weaker correlation with Omicron-specific neutralizing antibody titers than with those against the wildtype. In contrast, the sensitivity of nucleocapsid-protein-specific immunoassays was similar in wildtype and Omicron-infected subjects. In summary, the antigenic changes in the Omicron S lead to reduced immunoreactivity in the current commercial S- and RBD-specific antibody assays, impairing their diagnostic performance. Importance This study demonstrates that the antigenic changes of the SARS-CoV-2 Omicron variant affect test results from commercial Spike- and RBD-specific antibody assays, significantly diminishing their sensitivities and diagnostic abilities to assess neutralizing antibodies.

Additional heterologous versus homologous booster vaccination in immunosuppressed patients without SARS-CoV-2 antibody seroconversion after primary mRNA vaccination: a randomized controlled trial (preprint)

Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2)-induced coronavirus disease 2019 (COVID-19) has led to exponentially rising mortality, particularly in immunosuppressed patients, who inadequately respond to conventional COVID-19 vaccination. In this blinded randomized clinical trial (EudraCT 2021-002348-57) we compare the efficacy and safety of an additional booster vaccination with a vector versus mRNA vaccine in non-seroconverted patients. We assigned 60 patients under rituximab treatment, who did not seroconvert after their primary mRNA vaccination with either BNT162b2 (Pfizer-BioNTech) or mRNA-1273 (Moderna), to receive a third dose, either using the same mRNA or the vector vaccine ChAdOx1 nCoV-19 (Oxford-AstraZeneca). Patients were stratified according to the presence of peripheral B-cells. The primary efficacy endpoint was the difference in the SARS-CoV-2 antibody seroconversion rate between vector (heterologous) and mRNA (homologous) vaccinated patients by week four. Key secondary endpoints included the overall seroconversion and cellular immune response; safety was assessed at weeks one and four. Seroconversion rates at week four were comparable between vector (6/27 patients, 22%) and mRNA (9/28, 32%) vaccine (p=0.6). Overall, 27% of patients seroconverted; specific T-cell responses were observed in 20/20 (100%) vector versus 13/16 (81%) mRNA vaccinated patients. Newly induced humoral and/or cellular responses occurred in 9/11 (82%) patients. No serious adverse events, related to immunization, were observed. This enhanced humoral and/or cellular immune response supports an additional booster vaccination in non-seroconverted patients irrespective of a heterologous or homologous vaccination regimen.

The comparability of Anti-Spike SARS-CoV-2 antibody tests is time-dependent: a prospective observational study (preprint)

ObjectivesVarious commercial anti-Spike SARS-CoV-2 antibody tests are used for studies and in clinical settings after vaccination. An international standard for SARS-CoV-2 antibodies has been established to achieve comparability of such tests, allowing conversions to BAU/ml. This study aimed to investigate the comparability of antibody tests regarding the timing of blood collection after vaccination. MethodsFor this prospective observational study, antibody levels of 50 participants with homologous AZD1222 vaccination were evaluated at 3 and 11 weeks after the first dose and 3 weeks after the second dose using two commercial anti-Spike binding antibody assays (Roche and Abbott) and a surrogate neutralization assay. ResultsThe correlation between Roche and Abbott changed significantly depending on the time point studied. Although 3 weeks after the first dose, Abbott provided values three times higher than Roche, 11 weeks after the first dose, the values for Roche were twice as high as for Abbott, and 3 weeks after the second dose even 5-6 times higher. ConclusionsThe comparability of quantitative anti-Spike SARS-CoV-2 antibody tests is highly dependent on the timing of blood collection after vaccination. Therefore, standardization of the timing of blood collection might be necessary for the comparability of different quantitative SARS-COV-2 antibody assays. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=175 SRC="FIGDIR/small/21262426v1_ufig1.gif" ALT="Figure 1"> View larger version (27K): org.highwire.dtl.DTLVardef@1e789daorg.highwire.dtl.DTLVardef@b83aforg.highwire.dtl.DTLVardef@1f270daorg.highwire.dtl.DTLVardef@1cf296c_HPS_FORMAT_FIGEXP M_FIG C_FIG

Initial SARS-CoV-2 vaccination response can predict booster response for BNT162b2 but not for AZD1222 (preprint)

Objectives Our objective was to determine whether SARS-CoV-2 antibody levels after the first dose can predict the final antibody response and whether this is dependent on the vaccine type. Methods 69 BNT162b2 (Pfizer/BioNTech) and 55 AZD1222 (AstraZeneca) vaccinees without previous infection or immunosuppressive medication were included. Anti-body levels were quantified 3 weeks after dose 1, in case of AZD1222 directly before boostering (11 weeks after dose 1) and 3 weeks after dose 2, with the Roche SARS-CoV-2 S total antibody assay. Results Pre-booster (BNT162b2: 80.6 [25.5-167.0] BAU/mL, AZD1222: 56.4 [36.4-104.8] BAU/mL, not significant) and post-booster levels (BNT162b2: 2,092.0 [1,216.3-4,431.8] BAU/mL, AZD1222: 957.0 [684.5-1,684.8] BAU/mL, p<0.0001) correlated well in BNT162b2 ({rho}=0.53) but not in AZD1222 recipients. Moreover, antibody levels after the first dose of BNT162b2 correlated inversely with age ({rho}=-0.33, P=0.013), whereas a positive correlation with age was observed after the second dose in AZD1222 recipients ({rho}=0.26, P=0.030). Conclusions In conclusion, our data suggest that antibody levels quantified by the Roche Elecsys SARS-CoV-2 S assay before the booster shot could infer post-booster responses to BNT162b2, but not to AZ1222. In addition, we found a vaccine-dependent effect on antibody responses, suggesting a possible link between vaccine response and vector immunity.

Spike protein antibodies mediate the apparent correlation between SARS-CoV-2 nucleocapsid antibodies and neutralization test results (preprint)

ObjectivesSARS-CoV-2 infection induces the formation of different antibodies. However, not all of which might prevent the virus from entering the cell, although their concentrations correlate with the titers of viral neutralization tests (NTs). Antibodies against the viral nucleocapsid (NC), e.g., can be classified as such. We aimed to prove the hypothesis that the apparent correlation between NC-antibody levels and NT-titers is mediated by simultaneously occurring antibodies against viral spike-protein components. MethodsWe included 64 individuals with previous SARS-CoV-2 infection (>14d after symptom onset). SARS-CoV-2 antibodies against the NC (Roche total antibody ECLIA, Abbott IgG CMIA) and spike-protein (Technozym RBD ELISA, DiaSorin S1/S2 CLIA) were measured, and neutralization tests were performed. The effect of spike-protein antibodies on the correlation between NC-antibodies and NT-titers was evaluated by partial correlation and mediation analyses. ResultsBoth tested assays assessing antibodies against the NC correlated significantly with NT titers: Abbott {rho}=0.742, P<0.0001; Roche {rho}=0.365, P<0.01. However, when controlling the rank correlations for the presence of RBD or S1/S2 antibodies, correlation coefficients dropped to {rho}=0.318/{rho}=0.329 (P<0.05/P<0.01), respectively for Abbott and vanished for Roche. As a result, only a maximum of 11% of NT titer variability could be explained by NC-antibody levels. ConclusionsOur data suggest that the apparent correlation between NC antibodies and NT titers is strongly mediated by co-occurring RBD antibody concentrations. To avoid falsely implied causal relationships, all correlation analyses of non-spike-associated antibody assays and neutralization assays should include a partial correlation analysis to exclude a possible mediator effect of spike-associated antibodies.

Anti-Spike protein assays to determine post-vaccination antibody levels: a head-to-head comparison of five quantitative assays (preprint)

Background Reliable quantification of the antibody response to SARS-CoV-2 vaccination is highly relevant for identifying possible vaccine failure and estimating the time of protection. Therefore, we aimed to evaluate the performance of five different Anti-SARS-CoV-2 antibody assays regarding the quantification of anti-spike (S) antibodies induced after a single dose of BNT162b2. Methods Sera of n=69 SARS-CoV-2 naive individuals 21+/-1 days after vaccination with BNT162b2 (Pfizer/BioNTech) were tested using the following quantitative SARS-CoV-2 antibody assays: Roche S total antibody, DiaSorin trimeric spike IgG, DiaSorin S1/S2 IgG, Abbott II IgG, and Serion/Virion IgG. Test agreement was assessed by Passing-Bablok regression. Results were further compared to the percent inhibition calculated from a surrogate virus neutralization test (sVNT) by correlation and ROC (receiver-operating-characteristics) analysis. Results Individual values were distributed over several orders of magnitude for all assays evaluated. Although the assays were in good overall agreement (rho=0.80-0.94), Passing-Bablok regression revealed systematic and proportional differences, which could not be eliminated by converting the results to BAU/mL as suggested by the manufacturers. 7 (10%) individuals had a negative sVNT results (i.e. <30% inhibition). These samples were reliably identified by most assays and yielded low binding antibody levels (ROC-AUCs 0.84-0.93). Conclusions Although all assays evaluated showed good correlation, readings from different assays were not interchangeable, even when converted to BAU/mL using the WHO international standard for SARS-CoV-2 immunoglobulin. This highlights the need for further standardization of SARS-CoV-2 serology.

A comprehensive antigen production and characterization study for easy-to-implement, highly specific and quantitative SARS-CoV-2 antibody assays (preprint)

Antibody tests are essential tools to investigate humoral immunity following SARS-CoV-2 infection. While first-generation antibody tests have primarily provided qualitative results with low specificity, accurate seroprevalence studies and tracking of antibody levels over time require highly specific, sensitive and quantitative test setups. Here, we describe two quantitative ELISA antibody tests based on the SARS-CoV-2 spike receptor-binding domain and the nucleocapsid protein. Comparative expression in bacterial, insect, mammalian and plant-based platforms enabled the identification of new antigen designs with superior quality and high suitability as diagnostic reagents. Both tests scored excellently in clinical validations with multi-centric specificity and sensitivity cohorts and showed unprecedented correlation with SARS-CoV-2 neutralization titers. Orthogonal testing increased assay specificity to 99.8%, thereby enabling robust serodiagnosis in low-prevalence settings. The inclusion of a calibrator permits accurate quantitative monitoring of antibody concentrations in samples collected at different time points during the acute and convalescent phase of COVID-19.

Increasing both specificity and sensitivity of SARS-CoV-2 antibody tests by using an adaptive orthogonal testing approach (preprint)

Background SARS-CoV-2 antibody tests have undergone a remarkable improvement in performance. However, due to the low seroprevalence in several areas, very high demands are made on their specificity. Furthermore, the low antibody-response in some individuals requires high test sensitivity to avoid underestimating true seroprevalence. Optimization of testing has been reported through lowering manufacturer cut-offs to improve SARS-CoV-2 assay sensitivity or by combining two tests to improve specificity at the cost of sensitivity. However, these strategies have thus far been used in isolation of each other. Methods To increase sensitivity, cut-offs of three commercially available SARS-CoV-2 automated assays (Roche, Abbott, and DiaSorin) were reduced according to published values in a pre-pandemic specificity cohort (n=1117) and a SARS-CoV-2 positive cohort (n=64). All three testing systems were combined in an orthogonal approach with a confirmatory test, which was one of the remaining automated assays or one of two commercial ELISAs directed against the spike protein receptor binding-domain (RBD) or the nucleocapsid antigen (NP). Results The modified orthogonal test strategy resulted in an improved specificity of at least 99.8%, often even 100%, in all 12 tested combinations with no significant decline in sensitivity. In our cohort, regardless of whether the assays were used for screening or confirmation, combining Roche and Abbott delivered the best overall performance (+~10% sensitivity compared to the single tests and 100% specificity). Conclusion Here we propose a novel orthogonal assay strategy that approaches 100% specificity while maintaining or even significantly improving the screening test's sensitivity.

Side by side comparison of three fully automated SARS-CoV-2 antibody assays with a focus on specificity (preprint)

Background: In the context of the COVID-19 pandemic, numerous new serological test systems for the detection of anti-SARS-CoV-2 antibodies have become available quickly. However, the clinical performance of many of them is still insufficiently described. Therefore we compared three commercial, CE-marked, SARS-CoV-2 antibody assays side by side. Methods: We included a total of 1,154 specimens from pre-COVID-19 times and 65 samples from COVID-19 patients ([≥]14 days after symptom onset) to evaluate the test performance of SARS-CoV-2 serological assays by Abbott, Roche, and DiaSorin. Results: All three assays presented with high specificities: 99.2% (98.6-99.7) for Abbott, 99.7% (99.2-100.0) for Roche, and 98.3% (97.3-98.9) for DiaSorin. In contrast to the manufacturers' specifications, sensitivities only ranged from 83.1% to 89.2%. Although the three methods were in good agreement (Cohen's Kappa 0.71-0.87), McNemar's test revealed significant differences between results obtained from Roche and DiaSorin. However, at low seroprevalences, the minor differences in specificity resulted in profound discrepancies of positive predictability at 1% seroprevalence: 52.3% (36.2-67.9), 77.6% (52.8-91.5), and 32.6% (23.6-43.1) for Abbott, Roche, and DiaSorin, respectively. Conclusion: We find diagnostically relevant differences in specificities for the anti-SARS-CoV-2 antibody assays by Abbott, Roche, and DiaSorin that have a significant impact on the positive predictability of these tests.