Baku, July 23, AZERTAC Researchers in Sweden have found that, even though virus-specific T cells against several variants of SARS-CoV-2 disappeared 10-12 weeks after infection, a group of highly specialized T cells designed to facilitate elimination of infected cells remained active in the blood even after a long follow-up period, according to Biocompare.The team’s findings, published recently in Proceedings of the National Academy of Sciences, may explain the reduced risk of severe disease and mortality among patients who have become re-infected with SARS-CoV-2. It also assuages concerns that immunity does not persist over time and could translate into severe COVID-19 upon re-infection.The team at the University of Gothenburg and Sahlgrenska University Hospital collected 81 blood samples from hospital staff members who had contracted mild COVID-19 in the first year of the pandemic and uninfected controls. The researchers studied T cell reactivity to an inner part of SARS-CoV-2 (the virus nucleocapsid) thus capturing T cell responses that only occur after a natural infection.It was observed that a subgroup of specialized T cells (Th1 cells) that promote destruction of virus-infected cells were active for at least 20 months after natural COVID-19. The infected patients also harbored several other types of T cells that reacted with SARS-CoV-2. These latter T cells disappeared from blood approximately 2 months after recovery from infection.The blood samples were exposed to more than 100 peptides from the nucleocapsid portion of SARS-CoV-2 virus. The researchers then analyzed which T cell mediators (cytokines) were produced by blood cells to determine the longevity of T cell reactivity after the infection.“While certain subsets of T cells disappear shortly after infection, highly specialized T cells (T helper 1 cells) remain stably present in blood to suggest that a vital aspect of protective immunity is functional years after COVID-19,” says Anna Martner, Associate Professor of immunology at the Sahlgrenska Academy. The results may explain why re-infection with SARS-CoV-2 only rarely translates into severe COVID-19, she adds.