Comparison of two assays to detect IgG antibodies to the receptor binding domain of SARS‑CoV‑2 as a surrogate marker for assessing neutralizing antibodies in COVID-19 patients.

BackgroundNeutralizing antibodies (NAbs) are important for protection against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) reinfection. In this study, two assays that are correlated with NAbs were compared: the haemagglutination test (HAT) and the surrogate virus neutralization test (sVNT).MethodsThe specificity of the HAT was compared with the sVNT, and the sensitivity and persistence of antibodies in patients with varying severity of illness was assessed in a cohort of 71 patients at 4-6 weeks and 13-16 weeks. The kinetics were assessed in the first, second, and third weeks in patients with varying severity of acute illness.ResultsThe specificity of the HAT was >99%, and sensitivity was similar to the sVNT. The levels of HAT were significantly and positively correlated with those of the sVNT (Spearman's r = 0.78, P < 0.0001). Patients with moderate and severe illness had higher HAT titres when compared to those with mild illness. Six of seven patients with severe illness had a titre of >1:640 during the second week of illness, whereas only five of 31 patients with a mild illness had a titre of >1:160 in the second week of illness.ConclusionsSince the HAT is a simple and very cheap assay to perform, it would be ideal to use as an indicator of NAbs in resource-poor settings.