Maintaining immunological memory to the SARS-CoV-2 virus during COVID-19 pandemic

Abstract

The question on the duration and effectiveness of post-infection vs post-vaccination SARS-CoV-2 immunity remains in the focus of numerous studies. The aim of the work was to examine the duration of maintained post-infection and post-vaccination SARS-CoV-2 immunity as well as formation of hybrid (vaccination after infection) and breakthrough (repeated disease or disease after vaccination) immunity in the context of an ongoing COVID-19 pandemic. 107 adults with mild or moderate COVID-19 318 months after the disease and 30 subjects vaccinated twice with the Sputnik V vaccine were examined 16 times. Antibodies against SARS-CoV-2 virus were determined by ELISA on the SARS-CoV-2-IgG quantitative-ELISA-BEST test systems. The antibody avidity was measured 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 using FACSCanto II cytometer. It was shown that in the group of convalescent and vaccinated subjects, the level of virus-specific antibodies decreased more deeply in individuals with initially high humoral response, but 9 months later the decrease slowed down and reached a plateau. The antibody avidity rose up 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 6 months later, but remained at detectable level. After revaccination, a significant increase in the level of antibodies, avidity up to 67.6% and cellular immunity returned to the initial level were 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 that 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 dynamic observation of four COVID-19 convalescents, it has been shown that in the context of the ongoing pandemic and active coronavirus mutation, natural boosting occurs both asymptomatically and as a result of a mild re-infection, which prevents disappearance of SARS-CoV-2 humoral and cellular immunity.