Patients infected with SARS-CoV-2 develop protective immune responses mediated by virus-specific T cells and antibodies shortly after infection. However, there is concern that immunity will wane, resulting in severe COVID-19 infection upon re-infection.
Anna Martner and colleagues from the University of Gothenburg publish two key results in the Proceedings of the National Academy of Sciences on July 12. (PNAS). Initially, numerous virus-specific T cell variations were discovered in blood soon after COVID-19 but subsequently disappeared within 10-12 weeks. However, in the blood of all previously SARS-CoV-2-infected individuals, a fraction of highly specialised T cells, which are meant to assist destroy infected cells, remained active. These T cells did not vanish or diminish even after an extended period of monitoring.
The findings might explain why people infected with SARS-CoV-2 again have a decreased risk of severe illness and death.
Researchers from the University of Gothenburg and Sahlgrenska University Hospital gathered 81 blood samples from hospital staff members who had moderate COVID-19 in the first year of the pandemic, as well as uninfected controls, during the first year of the epidemic. T cell reactivity to an inner component of SARS-CoV-2 (the virus nucleocapsid) was investigated, capturing T cell responses that occur only after a natural infection.
Specialized T cells
Blood samples were given over 100 peptides from the nucleocapsid component of the SARS-CoV-2 virus. The researchers next examined which T cell mediators (cytokines) were created by blood cells to determine the duration of T cell responsiveness following infection.
It was observed that a subgroup of specialized T cells (Th1 cells) that promote the destruction of virus-infected cells were active for at least 20 months after natural COVID-19. The infected patients also harboured 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.
“While certain subsets of T cells disappear shortly after an infection, highly specialized T cells (T helper 1 cells) remain stably present in the 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. These results may explain why re-infection with SARS-CoV-2 only rarely translates into severe COVID-19.