Comparison between neutralizing antibody titres and anti-RBD SARS-CoV2 IgG

ABSTRACT

Objectives: Serological assays for virus-specific antibody response are commonly employed to establish the immunological response to vaccines and correlates of protection. We aimed to establish the usefulness of immunoassays for the quantitation of IgG antibody response to SARS-CoV2 receptor binding domain (anti-RBD) in the prediction of neutralizing antibody response. Method(s) An observational study was performed, included 97 subjects with positive anti-RBD SARS-Cov2 IgG from Mauriziano hospital S.C. Laboratorio Analisi between December 2021 and June 2022 divided into two groups the first group (A, n=37) with positive nasal swab for SARS-CoV2 or presence of IgG direct to Nucleocapside antigens (anti-N > 0.30 AU/mL with SARSCoV2 IgG method, Abbott Diagnostics) and second group (B, n=60) with no evidence of late infection. All samples was tested for SARS-CoV2 neutralizing antibody titres (NAb, SGM Italia). Anti-RBD SARS-CoV2 IgG were measured with automated chemiluminescent immunoassay, the results were expressed in Binding Antibody Units (BAU/mL);NAb were performed with quantitative latex immunoturbidimetry method and the results were expressed in neutralizing percent (NAb %). Statistical analysis was performed with Analyze-it. The comparison between anti-RBD levels and neutralizing antibodies has been studied using correlation by Spearman method and receiver operating characteristics (ROC) curve analysis. Result(s) A significant correlation between anti-RBD IgG levels and NAb titres was found in both A and B groups (rs=0.90 and 0.91 respectively, p<0.0001). The 99% probability of hight NAb titer (>56%) was reached at 212 BAU/ml (A) and 238 BAU/mL (B) and the area under the receiver operating characteristic (ROC) curve was 0.99 (CI 0.98-1.0) and 1 in A and B respectively. Conclusion(s) Our study confirm that there is a hight degree of concordance between anti-RBD antibodies and NAb. Based on our data, a threshold of 220 BAU/mL is highly predictive of strong neutralizing antibody response. However these results sure need further confirmation on a higher number of samples.