According to a study published in Nature Reviews Immunology, people unexposed to Covid-19 may have some pre-existing immunity against the virus. However, the source and clinical relevance of the reactivity remains unknown.
The study, ‘Pre-existing immunity to SARS-CoV-2: the knowns and unknowns’, noted that people have some T-cell reactivity against SARS-CoV-2. The researchers said that it is important to define the specifics of these T-cells and assess their association with Covid-19 disease severity and vaccine responses.
According to the findings, lymphocytes, a subtype of a white blood cell in a vertebrate’s immune system, from 20-50 per cent of unexposed donors display significant reactivity to SARS-CoV-2 antigen peptide pools.
The study highlighted that pre-existing T-cell immunity to SARS-CoV-2 could be relevant because it could influence Covid-19 disease severity.
It is plausible that people with a high level of pre-existing memory CD4+ T cells that recognise SARS-CoV-2 could mount a stronger immune response upon exposure to SARS-CoV-2 and thereby limit disease severity.
The study mentioned that memory CD4+ and CD8+ T cells might also facilitate direct antiviral immunity in the lungs and nasopharynx early after exposure of the coronavirus.
Pre-existing CD4+ T-cell memory could also influence vaccination outcomes, leading to a faster or better immune response, particularly the development of neutralising antibodies, which generally depend on T-cell help.