Evaluation of serological anti-SARS-CoV-2 chemiluminescent immunoassays correlated to live virus neutralization test, for the detection of anti-RBD antibodies as a relevant alternative in COVID-19 large-scale neutralizing activity monitoring

Abstract

The Spike-Receptor Binding Domain (S-RBD) is considered the most antigenic protein in SARS-CoV-2 and probably the key player in SARS-CoV-2 immune response. Quantitative immunoassays may help establish an anti-RBD Abs threshold as an indication of protective immunity. Since different immunoassays are commercial, the standard reference method for the neutralizing activity is the live Virus Neutralization Test (VNT). In this study, anti-RBD IgG levels were detected with two chemiluminescent immunoassays in paucisymptomatic, symptomatic and vaccinated subjects, and their neutralizing activity was correlated to VNT titer, using SARS-CoV-2 original and British variant strains. Both immunoassays confirmed higher anti-RBD Abs levels in vaccinated subjects. Furthermore, despite different anti-RBD Abs median concentrations between the immunoassays, a strong positive correlation with VNT was observed. In conclusion, although the SARS-CoV-2 immune response heterogeneity, the use of immunoassays can help in large-scale monitoring of COVID-19 samples, becoming a valid alternative to VNT test for diagnostic routine laboratories.

Keywords: Anti-SARS-CoV-2 antibodies; COVID-19; Live virus neutralization test; SARS-CoV-2; Serological immunoassays.

Fig. 1

IgG anti-RBD antibodies concentrations detected by SNIBE and MINDRAY assays in the study cohorts.

Fig. 2

Distribution of IgG anti-RBD antibodies concentrations in the three groups.

Fig. 3

Spearman's rank correlation between SNIBE and MINDRAY results.

Fig. 4

Distribution of anti-RBD Abs levels correlated to neutralization titers. A) SNIBE results from original strain; B) MINDRAY results from original strain; C) SNIBE results from the British variant strain; D) MINDRAY results from the British variant strain. VNT titers are expressed as the serum dilution reciprocals.

Fig. 5

SARS-CoV-2 original strain live VNT ROC curves. A) SNIBE assay; B) MINDRAY assay.

Fig. 6

Spearman's rank correlation of IgG anti-RBD concentrations to VNT titers. A) SNIBE results from original strain; B) MINDRAY results from original strain; C) SNIBE results from the British variant strain; D) MINDRAY results from the British variant strain. Vaccinated subjects showed the strongest correlation for both SNIBE and MINDRAY assays (r = 0,8668 and r = 0,8436, respectively); symptomatic patients showed the least correlation (r = 0,6248 and r = 0,5546, respectively). Results were also confirmed using the British variant strain: r = 0,8732 (SNIBE) and r = 0,8690 (MINDRAY) in vaccinated group; r = 0,6522 (SNIBE) and r = 0,5452 (MINDRAY) in symptomatic patients' group.