Humoral immunity in reconvalescents and persons vaccinated against COVID-19: a comparative assessment of the main indices of the humoral immune response

Specific humoral immunity to SARS-CoV-2 develops due to the formation of neutralizing IgG, which can primarily block the receptor-binding domain of the viral S-protein. The duration of post-infection immunity, as well as avidity of circulating antibodies, play an important role in this process. The aim of this work was to evaluate the amounts of antibodies to SARS-CoV 2 S-protein, their avidity and neutralizing activity in the studied samples of the post-COVID patients versus vaccinated seropositive individuals. Materials and methods. A sample of 113 individuals was studied, which consisted of three experimental groups, i.e.: recovered, vaccinated, as well as recovered and vaccinated persons. Blood serum specimens of the individuals were studied for specific IgG to SARS-CoV-2, along with determination of their quantities (BAU/mL) using Vector-Best kits (Novosibirsk, Russia). The avidity index was determined using a kit manufactured by MedipalTech (Dubna, Russia). Neutralizing ability of the antibodies was assayed by means of ELISA with diagnostic kits from MedipalTech (Dubna, Russia), which resulted into percentage of neutralized S-proteins to RBD. Results. The average levels of IgG did not show significant differences between reconvalescents and vaccinated persons. However, both indicators were significantly lower than those from the groups who recovered from the disease and were vaccinated. A cyclic change in the numbers of antibodies was observed, along with most intensive drop in the level of immunoglobulins over first four months after the illness or vaccination. Despite initially similar levels of immune parameters in both groups, the decline of this index in “vaccinated” group was significantly higher than in the “recovered” group, thus allowing us to conclude that the amounts of specific antibodies in this group was shown to be decreased to zero levels as soon as by the 10^th month. IgG index among the «recovered and vaccinated» groups remained unchanged for the entire anamnestic period. Avidity index of the antibodies in vaccinated individuals was higher than in recovered individuals. Meanwhile, this index in both groups was characterized by stable increase over the observation period of 7 to 11 months. The highest levels of antibodies and their avidity were noted in the group of recovered and vaccinated individuals, due to the most complete activation of the immune system. A straight-line trend was revealed for the decreasing index of neutralizing activity during the considered time period. The overall pattern of thee results shows that the neutralizing activity of antibodies is largely determined by the amounts of SARS-CoV-2-specific immunoglobulins. Thus, the time dynamics of antibodies to SARS-CoV-2 in various groups of examined individuals was revealed. Direct correlation was established between the neutralizing activity and amounts of immunoglobulins, as well as the role of vaccination for increased avidity of antibodies.