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

background: Neutralizing antibodies (NAbs) are important for protection against COVID-19 re-infection. We compared two assays that are correlated with NAbs, the haemagglutination test (HAT) and surrogate virus neutralization test (sVNT). methods: The 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 cohort of 71 patients at 4 to 6 weeks and 13-16 weeks. The kinetics was assessed in first, second and third week in patients with varying severity of acute illness. results: The specificity of the HAT was >99%, and sensitivity was similar to lower than the sVNT. The levels of HAT significantly and positively correlated (Spearman's r= 0.78, p<0.0001) with those of the sVNT. Patients with moderate and severe illness had higher in the HAT titres compared to those with mild illness. 6/7 patients with severe illness had a titre of >1:640 during the second week of illness, whereas only 5/31 patients with mild illness had a titre of >1:160 in the second week of illness. conclusions: Since 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.