Robust cellular immunity to SARS-CoV-2 — the coronavirus that causes Covid-19 — appears to last for at least six months after infection, and white blood cell levels are higher in people who experienced symptoms, a small new study has found. 

Researchers from the UK Coronavirus Immunology Consortium and Public Health England studied 100 non-hospitalized people who had been infected with Covid-19, but experienced mild symptoms or remained asymptomatic, and found robust T cell responses to SARS-CoV-2 even six months after their infection. The study has not yet been peer-reviewed and is available as a preprint paper on bioRxiv.

Notably, the size of the T cell response was considerably higher (around 50 percent higher) in people who had experienced symptoms rather than those who didn’t have any symptoms.

“Cellular immunity is a complex but potentially very significant piece of the Covid-19 puzzle, and it’s important that more research be done in this area,” Dr Shamez Ladhani, a consultant epidemiologist at Public Health England and one of the study authors, said in a statement. 

“However, early results show that T-cell responses may outlast the initial antibody response, which could have a significant impact on Covid vaccine development and immunity research." 

There’s been a lot of attention on how quickly antibodies to Covid-19 last after infection, with a bunch of recent studies suggesting that antibodies may fade away within a matter of months. However, immunity is hugely complex and not just about antibodies. Along with immunity mediated by antibodies, the body’s immune response to pathogens also involves cellular immunity through the activation of phagocytes, cytokines, T cells, and other immune cells.

Although this study cannot tell us whether people are fully protected from reinfection, it does suggest a significant cellular immune response remains for a prolonged amount of time, which is welcome news. 

"While our findings cause us to be cautiously optimistic about the strength and length of immunity generated after SARS-CoV-2 infection, this is just one piece of the puzzle. There is still a lot for us to learn before we have a full understanding of how immunity to Covid-19 works," added Professor Paul Moss, study author and the UK Coronavirus Immunology Consortium lead at the University of Birmingham. 

There are some limitations to the research that should be noted. Firstly, this study involved just 100 people, which is a very small sample size for a disease that’s causing a global pandemic. Secondly, most of the participants were relatively young (mean age of 41) and healthy. It's not clear yet whether this effect will also be clearly seen in high-risk groups or people that fall severely ill with the disease. 

Independent experts reacting to the preprint paper have praised the research, noting its findings are “promising” and could help to guide some of the next steps towards controlling and treating the pandemic. 

“This excellent study provides strong evidence that T-cell immunity to SARS-CoV-2 may last longer than antibody immunity,” commented Professor Charles Bangham, Chair of Immunology at Imperial College London, who was not involved in the study.

“The data are consistent with previous observations on T-cell immunity to SARS – with SARS some patients had T-cells more than 10 years after infection, though we don’t yet know whether this will be the case with Covid-19.”