Patients infected with SARS-CoV-2 develop protective immune responses, mediated by virus-specific T cells and antibodies, soon after infection. However, there are concerns that immunity may not persist over time, which may result in severe COVID-19 upon reinfection.
In the July 12 issue of Proceedings of the National Academy of Sciences (PNAS), Anna Martner and co-authors from the University of Gothenburg report two main findings. First, several virus-specific T cell variants became detectable in the blood shortly after COVID-19, but disappeared strikingly after 10 to 12 weeks.
However, a group of highly specialized T cells designed to facilitate the elimination of infected cells remained active in the blood of all patients previously infected with SARS-CoV-2. These T cells did not disappear or decline even after long follow-up.
The findings may explain the reduced risk of severe disease and mortality in patients who have been reinfected with SARS-CoV-2.
Scientists from the University of Gothenburg and Sahlgrenska University Hospital took 81 blood samples from hospital staff who had contracted mild COVID-19 in the first year of the pandemic and from uninfected controls. The researchers studied the reactivity of T cells to an internal part of SARS-CoV-2 (the viral nucleocapsid), thereby capturing T cell responses that only occur after natural infection.
Blood samples were exposed to more than 100 peptides from the nucleocapsid part of the 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 responsiveness after infection.
A subset of specialized T cells (Th1 cells) that promote the destruction of virus-infected cells have been observed to be 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 last T cells disappeared from the blood about 2 months after the infection was cured.
“While some T cell subsets disappear shortly after infection, highly specialized T cells (T helper cells 1) 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 findings may explain why reinfection with SARS-CoV-2 only rarely results in severe COVID-19.