Comparative analysis of IgG levels in blood sera from patients with COVID-19, persons vaccinated by «Gam-COVID-Vac» and healthy donors before the pandemic

Introduction. Serum IgG measurement is used in the diagnosis and monitoring of many diseases. Therefore, it seems important to study the impact of coronavirus pandemic on IgG levels in population and the role of this parameter in COVID-19. The aim of this study was to compare mean IgG levels in sera obtained from 31 hospitalized COVID-19 patients, 30 healthy donors before pandemic and 34 donors vaccinated with «Sputnik V» (have not had COVID-19). Materials and methods. Total IgG was quantitated by two enzyme immunoassays (EIAs): «IgG Total-ELISA-BEST» kit certificated in Russia and homemade competitive EIA utilizing bispecific monoclonal antibodies (bsAbs) against human IgG (HIgG) and horseradish peroxidase (HRP). Results and discussion. The groups did not show differences in IgG levels (regardless of sex) with both methods giving comparable results. However, «IgG Total-ELISA-BEST» kit revealed statistically significant differences in mean serum IgG levels in subgroups of male patients depending on the levels of antibodies to viral RBD-antigen: below and above 400 BAU/ml. In the first subgroup (10 men) the mean serum IgG content was 14.3 ± 4.1 mg/mL, while in the second (6 men) — 6.9 ± 2.7 mg/mL. Conclusion. Sera obtained before pandemic contained the same mean IgG concentrations as sera from donors vaccinated with «Sputnik V» and COVID-19 patients. The relatively decreased mean IgG concentration was found only in COVID-19 male patients with anti-RBD antibodies levels above 400 BAU/ml. In light of literature data on association of decreased serum IgG with COVID-19 severity, it would be reasonable to further compare larger groups, taking into account clinical differences. The possibility of using bsAbs for human Ig measurements by competitive EIA was demonstrated.