Cellular (T cell) immunity against SARS-CoV-2 is likely to be present within most adults six months after first catching the virus, suggests a study published in Nature Immunology. The research from UK Coronavirus Immunology Consortium (UK-CIC), Public Health England, and the Manchester University NHS Foundation Trust found robust T cell responses to small pieces of the SARS-CoV-2 virus at this timepoint in all participants included in the study, who had experienced either asymptomatic or mild/moderate COVID-19 infection.
An important question is whether having previously been infected with SARS-CoV-2 means you are less likely to be reinfected, and if so for how long. The immune system is extremely complex and there are many ways it can generate immunity to a disease after infection. This study examines how T cells might contribute to immunity against SARS-CoV-2 for a considerable amount of time after first being infected.
The researchers collected serum and blood samples from a group of more than 2,000 clinical and non-clinical healthcare workers, 100 of whom tested positive for SARS-CoV-2 in March/April 2020. All 100 people experienced either mild/moderate symptoms (56 people) or were asymptomatic (44 people) and none were hospitalised for COVID-19. Antibody levels were checked each month using serum samples, and T cell (cellular) responses were checked after 6 months using blood samples. The researchers also looked at different aspects of the T cell response including comparing how they responded to different proteins from SARS-CoV-2.
T cells responses to SARS-CoV-2 were present in all 100 people six months after infection. The cellular immune response seen targeted a range of proteins from the virus, including the Spike protein that is targeted by many COVID-19 vaccines. However, there were also immune responses to other viral proteins, such as nucleoprotein, suggesting they could be used in future vaccine protocols. These findings suggest that a robust cellular memory against the virus is maintained for at least six months.
People who had experienced symptoms at the time of infection had 50% higher T cell responses, compared to those who were asymptomatic. Further research will be needed to understand this effect in more detail. It is possible that heightened cellular immunity could provide greater protection against re-infection in people who had symptoms during their first infection, or that people infected but without symptoms are simply able to fight off the virus without the need for a large immune response.
Antibodies are also an important part of immune defence and cellular immunity was strongly linked with the peak level of the antibody response. Furthermore, larger cellular responses appeared to protect against antibody ‘waning’ over time, again suggesting the need to ensure that cellular immune responses are produced following vaccination.
Overall, these findings indicate a strong cellular immune response against SARS-CoV-2 at six months post-infection. These findings will feed into our understanding of how immunity to SARS-CoV-2 works, and will help inform future vaccine strategies. Further research is now needed to assess whether this immune response is maintained over the longer term and to better understand how strength of cellular immune response corresponds to likelihood of reinfection.