Simple ELISA Methods to Estimate Neutralizing Antibody Titers to SARS-CoV-2: IgG Quantification, the Avidity Index, and the Surrogate Virus Neutralization Test

Author(s): Victor Manuylov, Inna Dolzhikova, Alexandra Kudryashova, Bogdan Cherepovich, Anna Kovyrshina, Anna Iliukhina, Olga Kharchenko, Maria Semashko, Artem Tkachuk, Vladimir Gushchin, Olga Borisova

A total of 104 sera sampled from vaccinated (“Sputnik V”, Russia) volunteers were tested in parallel to determine different markers of humoral immunity against SARS-CoV-2. Testing was conducted using the neutralizing antibodies titer (NtAb) in the virus neutralization assay (VNA), the IgG to RBD in the quantitative ELISA (BAU/ml), the avidity index (AI) of the IgG to RBD (also in ELISA), and the titers of “neutralizing antibodies” which can block an interaction between human ACE2 and the viral RBD proteins in a competitive ELISA surrogate virus neutralization test (sVNT). The correlation coefficients between the quantitative results in the tried ELISA assays and the “true” NtAb titers in the VNA were high, with the following values: r=0.86 for BAU/ml (95% confidence interval, CI: 0.80-0.91, p<0.0001); 0.54 for the AI (95% CI: 0,38-0,67, p<0.0001); and 0.84 for the sVNT titer (95% CI:0.79-0.90). Additionally, it was found that the multiplicative index of BAU/ml × AI (which corresponds to the concentration of the high-affinity fraction of the total IgG to the RBD) gives a maximum correlation to the NtAb titers (r=0.89, 95% CI: 0.84-0.92, p<0.0001). The sensitivity and specificity, respectively, of the tested ELISA assays in recognizing the supposedly protective NtAb titer of ≥1:160 were the following: for the BAU/ml – 87.7±8% and 97.4±5%; for the (BAU/ml × AI) index – 92.3±6.5% and 97.4±5%; for the sVNT—95.4±5.1% and 89.7±9.5%. This confirms that the tested ELISA technologies have potential as a safe and cheap alternative to the classical VNA for assessing the protective force of patient immunity against SARS-CoV-2 infection in routine clinical practice.