The question of the duration and effectiveness of post-infection immunity to SARS-CoV-2 and its comparison with post-vaccination remains at the center of study by many researchers. The aim of the work was to study the duration of maintaining post-infection and post-vaccination immunity to the SARS-CoV-2 virus, as well as the formation of hybrid (vaccination after infection) and breakthrough (repeated disease or disease after vaccination) immunity in the context of an ongoing pandemic. 107 adults who had mild or moderate COVID-19 3-18 months after the disease and 30 people vaccinated twice with the Sputnik V vaccine were examined 1-6 times. Antibodies to the SARS-CoV-2 virus were determined by ELISA on the SARS-CoV-2-IgG quantitative-ELISA-BEST test systems. The avidity of antibodies was determined by additional incubation with and without denaturing solution. Mononuclear cells were isolated from blood by gradient centrifugation, incubated with and without coronavirus S-protein for 20 hours, stained with fluorescently labeled antibodies, and the percentage of CD8highCD107a+ was counted on a FACSCanto II cytometer. It was shown that in the group of reconvalescent and vaccinated, the level of antibodies specific to the virus decreased more pronounced in individuals with an initially high humoral response, but after 9 months the decrease slowed down and reached a plateau. The avidity of antibodies rose to 50% and persisted for 18 months. Cellular immunity in recovered patients did not change for 1.5 years, while in vaccinated patients it gradually decreased after 6 months, but remained at a detectable level. After revaccination of the vaccinated, a significant increase in the level of antibodies, avidity to 67.6% and cellular immunity to the initial level was noted. Hybrid immunity turned out to be significantly higher than post-infection and post-vaccination immunity. The level of antibodies increased to 1218.2 BAU/ml, avidity to 69.85%, and cellular immunity to 9.94%. Breakthrough immunity was significantly higher than after the first disease. The level of antibodies rose to 1601 BAU/ml, avidity - up to 81.6%, cellular immunity - up to 13.71%. Using the example of dynamic observation of four COVID-19 reconvalescents, it has been shown that in the context of the ongoing pandemic and active mutation of the coronavirus, natural boosting occurs both asymptomatically and as a result of a mild re-infection, which prevents the disappearance of humoral and cellular immunity specific to SARS-CoV -2.

The SARS-CoV-2 virus that caused the COVID-19 pandemic is related to the SARS-CoV-1 and MERS coronaviruses, which were the cause of epidemics in 2003 and 2012. Antibodies in patients with COVID-19 appear 7-14 days after the onset of symptoms and gradually increase. As the COVID-19 pandemic continues, it is difficult to say how long the immunological memory to the SARS-CoV-2 virus will last. The aim of this study was to study the ratio of humoral and cellular immunity to the S-protein of the SARS-CoV-2 virus in COVID-19 convalescents. The study involved 60 adults with mild to moderate COVID-19 2 to 12 months prior to the examination. The control group consisted of 15 adults who did not have COVID-19 and were not vaccinated against this infection. Specific antibodies to the SARS-CoV-2 virus were determined by ELISA using the SARS-CoV-2-IgG-ELISA-BEST kit. To determine the specific IgG and IgA subclasses, the anti-IgG conjugate from the kit was replaced with a conjugate against the IgG subclasses and IgA. Additional incubation with or without denaturing urea solution was used to determine the avidity of antibodies. Mononuclear cells were isolated from blood by gradient centrifugation, incubated with or without coronavirus S antigen for 20 hours, stained with fluorescently labeled antibodies, and the percentage of CD8highCD107a were counted on a flow cytometer Canto II. In the control group, neither humoral nor cellular immunity to the S-protein of the SARS-CoV-2 was found. In the group of those who had recovered, the level of IgG antibodies to the S-protein of the SARS-CoV-2 virus varies greatly and was not strictly associated with the duration of the disease, 57% of COVID-19 patients had a high level of humoral response, and 43% - low level. The correlation between the levels of specific IgG and IgA was r = 0.43. The avidity of antibodies increased over time after the disease, amounting to 49.9% at a period of 6-12 months. There were no specific IgG subclasses IgG2 and IgG4, and the percentage of IgG1 increased over time and amounted to 100% after 6-12 months after illness. 50% of the examined had high cellular immunity, and the same number - low, no correlations with the level of humoral immunity were found. We identified 4 combinations of humoral and cellular immunity to the S-protein of the SARS-CoV-2: high humoral and cellular, low humoral and cellular, high humoral and low cellular, and vice versa, low humoral and high cellular immunity.