Abstract
BACKGROUND: The effector capabilities of humoral immunity are determined not only by the amount of specific antibodies produced in response to an antigenic effect, but also by their qualitative characteristics, which include avidity ― the total strength of binding to the antigen, which determines the duration and effectiveness of post-infectious immunity to SARS-CoV-2.
 AIM: Is a selective study of the quantity and avidity of IgG antibodies to SARS-CoV-2 over time among medical workers of a temporary infectious diseases hospital in Kazan ― convalescents of COVID-19, during the period from July 2020 to July 2021.
 MATERIALS AND METHODS: Determination of IgG to the S antigen of SARS-CoV-2 by ELISA was carried out using the test system SARS-CoV-2-IgG quantitative-ELISA-BEST (Vector-Best, Russia) and expressed in BAU/ml (binding antibody units). Antibody avidity was determined using a 4.0 M urea solution and expressed as avidity indices. 1, 4 and 7 months after COVID-19 asymptomatic (n=34); mild severity (n=42); moderate severity (n=29); reinfected (n=34). When statistically processing the data, descriptive statistics methods and the Wilcoxon matched data test were used. Differences were considered significant at p 0.05.
 RESULTS: IgG avidity to SARS-CoV-2 depended on the severity of COVID-19. The highest rates of avidity indices were found in the group of those who had a moderate form of COVID-19. If in mild and asymptomatic forms there was a parallel decrease in avidity indices and IgG titer, then in moderate forms an increase in antibody titer was accompanied by a decrease in their avidity 4 months after the infection. 7 months after seroconversion, the IgG level decreased almost twofold, both in mild, asymptomatic and moderate forms. In the group of medical workers who had COVID-19 repeatedly, the initially low levels of avidity indices and antibody titers increased in parallel, while avidity indices after 7 months did not decrease, but remained high. Differences in avidity indices determined the subsequent formation of different trends in the development of the humoral immune response, which were mainly characterized by an uneven decrease in IgG and persistence of IgM antibodies for more than 1 month.
 CONCLUSIONS: The research results expand the understanding of the mechanisms of formation of the humoral immune response and the avidity of IgG antibodies against SARS-CoV-2 in the risk group ― medical workers. The level of humoral immunity decreases in the first six months and varies depending on the severity of COVID-19. The data obtained can be used to identify categories of increased risk of SARS-CoV-2 infection among healthcare workers, make decisions about immunorehabilitation and revaccination against COVID-19.
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