RESUMO
The virus neutralization test (VNT) is the reference for the assessment of the functional ability of neutralizing antibodies (NAb) to block SARS-CoV-2 entry into cells. New competitive immunoassays measuring antibodies preventing interaction between the spike protein and its cellular receptor are proposed as surrogate VNT (sVNT). We tested three commercial sVNT (a qualitative immunochromatographic test and two quantitative immunoassays named YHLO and TECO) together with a conventional anti-spike IgG assay (bioMerieux) in comparison with an in-house plaque reduction neutralization test (PRNT50) using the original 19A strain and different variants of concern (VOC), on a panel of 306 sera from naturally-infected or vaccinated patients. The qualitative test was rapidly discarded because of poor sensitivity and specificity. Areas under the curve of YHLO and TECO assays were, respectively, 85.83 and 84.07 (p-value >0.05) using a positivity threshold of 20 for PRNT50, and 95.63 and 90.35 (p-value =0.02) using a threshold of 80. However, the performances of YHLO and bioMerieux were very close for both thresholds, demonstrating the absence of added value of sVNT compared to a conventional assay for the evaluation of the presence of NAb in seropositive subjects. In addition, the PRNT50 assay showed a reduction of NAb titers towards different VOC in comparison to the 19A strain that could not be appreciated by the commercial tests. Despite the good correlation between the anti-spike antibody titer and the titer of NAb by PRNT50, our results highlight the difficulty to distinguish true NAb among the anti-RBD antibodies with commercial user-friendly immunoassays.
RESUMO
With the availability of vaccines, commercial assays detecting anti-SARS-CoV-2 antibodies (Ab) evolved towards quantitative assays directed to the spike glycoprotein or its receptor binding domain (RBD). The main objective of the present study was to compare the Ab titers obtained with quantitative commercial binding Ab assays, after 1 dose (convalescent individuals) or 2 doses (naive individuals) of vaccine, in healthcare workers (HCW). Antibody titers were measured in 255 sera (from 150 HCW) with 5 quantitative immunoassays (Abbott RBD IgG II quant, bioMerieux RBD IgG, DiaSorin Trimeric spike IgG, Siemens Healthineers RBD IgG, Wantai RBD IgG). One qualitative total antibody anti RBD detection assay (Wantai) was used to detect previous infection before vaccination. The results are presented in binding Ab units (BAU)/mL after application, when possible, of a conversion factor provided by the manufacturers and established from a World Health Organization (WHO) internal standard. There was a 100% seroconversion with all assays evaluated after two doses of vaccine. With assays allowing BAU/ml correction, Ab titers were correlated (Pearson correlation coefficient, {rho}, range: 0.85-0.94). The titer differences varied by a mean of 10.6% between Siemens and bioMerieux assays to 60.9% between Abbott and DiaSorin assays. These results underline the importance of BAU conversion for the comparison of Ab titer obtained with the different quantitative assays. However, significant differences persist, notably, between kits detecting Ab against the different antigens. A true standardization of the assays would be to include the International Standard in the calibration of each assays to express the results in IU/mL.
RESUMO
ObjectivesWe evaluated widely-used SARS-CoV-2 serological tests and their potential association with virus neutralization test (VNT) in a cohort of mild COVID-19 patients. MethodsA total of 439 specimens were longitudinally collected from 76 healthcare workers with RT-PCR-confirmed mild COVID-19. Nine serological assays developed by leading global companies (Abbott, DiaSorin, Siemens, Bio-Rad, Wantai, bioMerieux, Euroimmun) were assessed. For each test the sensitivity to detect SARS-CoV-2 antibodies was determined weekly after symptom onset. Correlation and concordance were assessed using the Spearman and Cohens Kappa coefficients, respectively. Positive percent agreement and negative percent agreement (NPA) with the VNT were also determined. ResultsThe Wantai Total Ab assay targeting the receptor binding domain (RBD) within the S protein presented the best sensitivity at different times during the course of disease. The best correlation between antibody level and neutralizing antibody titer was found with the Euroimmun S1-based IgA assay (Spearman coefficient [95%CI]: 0.71 [0.61-0.79]). A moderate concordance (Kappa [95%CI]: 0.43[0.23-0.63]) as well as the lowest NPA (33%) was found between the Wantai Total Ab assay and the VNT. Compared to the Wantai Total Ab assay, other total Ab or IgG assays targeting the S or the RBD (bioMerieux, DiaSorin, Siemens,) were more concordant with the VNT (Kappa>0.7 for the three tests) and had a higher NPA (range: 90% to 97%). ConclusionsAlthough some assays presented a better concordance with VNT than others, the present findings emphasize that commercialized serological tests including those targeting the RBD cannot substitute VNT for the assessment of functional antibody response.
