Clinical laboratory diagnostics of antibodies to Sars-Cov-2: from a QR code to the reality

Background: The immune response to SARS-CoV-2 includes the production of specific immunoglobulins to protein antigens of SARS-CoV-2. Depending on the type and level of immunoglobulins, it is possible to assess the stage of the disease and evaluate the effectiveness of vaccination. The main approach to the determination of immunoglobulins to SARS-CoV-2 in human biological fluids is enzyme-linked sorbent immunoassay. Its data, in particular, are used to issue an electronic COVID-19 certificate with a QR code. However, the qualitative and quantitative composition of immunoglobulins for a QR code is not officially regulated. Aim: measuring the immunoglobulins’ level in the human blood serum with different types of immunity to the new coronavirus infection (COVID-19) to select the most informative indicators of protective immunity. Methods: The study included 76 blood serum samples from male and female volunteers (age, 18 to 50 y.o.) in compliance with the ethical standards. The detection of IgA, IgM, IgG (total to different regions of SARS-CoV-2, S-protein IgG and RBD-fragment IgG), IgG avidity, and the level of the SARS-CoV-2 N-antigen was performed by enzyme-linked immunosorbent assay (ELISA) using commercially available reagent kits. Results: The indicators of the level of antibodies (both "protective" IgG and IgA of the initial phase of infection) are most pronounced in persons who have been vaccinated and have had COVID-19, and least pronounced in unvaccinated people. For recovered unvaccinated individuals, the level of total “protective” antibodies and IgG to the S-protein, including the RBD fragment, is the lowest; the avidity of IgG is lower than that in the other groups, too. The IgG avidity in vaccinated patients is higher than that in recovered ones. It should be noted that there were no differences in the level of both total IgG to SARS-CoV-2, to the S-protein and to the RBD-fragment of the S-protein for recovered and vaccinated individuals. Conclusion: The analysis of COVID-19 immunoglobulins indicates a different profile of the humoral immune response following vaccination and previous infection with COVID-19. To quickly assess the immune response to previous and current COVID-19 infection, as well as to detect the post-vaccination immunity, it is advisable to use the total level of IgG to SARS-CoV-2. For deeper assessment of protective immunity and production of protective antibodies, it is better to evaluate the quantitative content of IgG to the S protein and its RBD fragment. The equal level of IgA in the experimental groups indicates an ongoing interaction with SARS CoV-2 in the population. Thus, the electronic COVID-19 certificate is of little use when it is formed by only one of the indicators without taking into account the rest.