Immunological memory to SARS-CoV-2 S protein persists 4 years after the disease

Ending of COVID-19 pandemic does not exclude subsequent breakthrough infections caused by SARS-CoV-2 mutant strains. The rate of SARS-CoV-2 mutations increased with emerging omicron strain and exceeds those of the influenza virus. It remains unclear what IgG antibody levels are able to protect against new mutant SARS-CoV-2 strains, and how long the immune protection will last. The objective of this study was to monitor the maintenance of humoral and cellular immunity to SARS-CoV-2 viral antigens over 4 years after the infection. Thirty-two adult reconvalescents after COVID-19 were annually examined for humoral and cellular immunity markers to the SARS-CoV-2 S protein. Humoral immunity was assessed by ELISA; cellular immunity was evaluated by expression of CD107a on CD8hi lymphocytes after recognition of S protein antigens. A four-year observation of a group of patients who recovered from COVID-19 in 2020 (SARS-CoV-2, Wuhan strain) and were in contact with a novel, freely circulating mutant VoCs showed that, 1 year later, all subjects retained the IgG antibodies to S protein, mainly the IgG1 subclass, but the antibody avidity index barely exceeded 50%. After a breakthrough infection caused by the omicron strain, the level of IgG antibodies to S protein increased significantly, along with sufficient increase of antibody avidity. The IgG2, IgG3, and IgG4 antibodies to S protein occured in the spectrum of subclasses. The level of specific IgA decreased 1 year after the disease against their level after the primary disease. However, it was significantly increased to 4-5 PR after breakthrough infections. Cellular immunity to the SARS-CoV-2 S protein was detected in all subjects at 1 year after the primary disease. After repeated infection with the omicron strain, it increased significantly and remained at this level for the next year. By 4 years, it decreased to the level that was a year after the disease. Hence, humoral and cellular immunity to S protein does not fade away, but continues to persist, maturate, being maintained at a sufficient level. Upon exposure to a new VoC, it allows to endure such a meeting either asymptomatically, or as a mild clinical infection. In view of frequent mutations in the S protein, the role of T-cell responses in anti-infectious protection seems to increase significantly. When developing new vaccines, one should rely on development of cellular immunity.