CHANGES IN THE AVIDS OF IGG ANTIBODIES TO THE S PROTEIN OF SARS-COV-2 AFTER CORONAVIRUS INFECTION IN MEDICAL WORKERS OF A TEMPORARY INFECTIONS HOSPITAL

AIMS: is a selective study of the quantity and avidity of IgG antibodies to SARS-CoV-2 over time among medical workers (HCWs) of a temporary infectious diseases hospital in Kazan - convalescents of COVID-19, during the period from July 2020 to July 2021. 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 (JSC Vector-Best, Russia) and expressed in BAU\ml. Antibody avidity was determined using a 4.0 M urea solution and expressed as avidity indices (AI) 1, 4 and 7 months after COVID-19. RESULTS: IgG avidity to SARS-CoV-2 depended on the severity of COVID-19. The highest rates of IA 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 AI 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 HCWs who had COVID-19 repeatedly, the initially low levels of AI and antibody titers increased in parallel, while AI after 7 months did not decrease, but remained high. Differences in AI 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 Ig M 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 - MR. 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.