T-cell immunity against Covid-19 is likely to be present within most adults six months after primary infection, according to a study.
The research from Public Health England (PHE) and the UK Coronavirus Immunology Consortium (UK-CIC) demonstrated robust T-cell responses to Covid-19 peptides at six months in all participants following asymptomatic, mild or moderate infection.
Professor Paul Moss, UK-CIC lead and professor of haematology at the University of Birmingham, described the new data as “reassuring, potentially even encouraging” but said it does not mean people cannot get re-infected.
The research was an observational study and has not yet been peer-reviewed.
As part of UK-CIC, researchers from the University of Birmingham, PHE and NIHR Manchester Clinical Research Facility collected serum and blood samples from a cohort of more than 2,000 clinical and non-clinical healthcare workers.
This included 100 individuals – 23 men and 77 women – who tested sero-positive for Sars-CoV-2 in March or April, with an average age of 41 (range 22-65).
All 100 individuals experienced either mild or moderate symptoms, or were asymptomatic (56 versus 44 people) and none was admitted to hospital.
Serum samples were collected monthly to measure antibody levels, and blood samples were taken after six months to assess the cellular (T-cell) response.
A range of analyses were carried out to assess different aspects of the T-cell response, including the magnitude of response and the response to different proteins from Sars-CoV-2.
Carrying out these cellular analyses is much more complex than antibody studies – but the authors said this study of 100 individuals is one of the largest in the world in this field to date.
The study found T-cell responses were present in all individuals at six months after Sars-CoV-2 infection.
The cellular immune response was directed against a range of proteins from the virus, including the Spike protein that is being used in most vaccine studies.
However, comparable immunity was present against additional proteins, such as nucleoprotein, which suggests that these may be of value for incorporation in future vaccine protocols.
This indicates that a robust cellular memory against the virus persists for at least six months.
The size of T-cell response differed between individuals, being considerably higher (50%) in people who had experienced symptomatic disease at the time of infection six months previously.
The authors said further research will be needed to determine the significance of this finding.
They believe it is possible that heightened cellular immunity might provide increased protection against re-infection in people with initial symptomatic infection, or that asymptomatic individuals are simply able to fight off the virus without the need to generate a large immune response.
Larger cellular responses appeared to protect against antibody waning over time, suggesting the need to ensure that cellular immune responses are elicited in vaccine regimens.
The authors said the findings will feed not only into the understanding of how immunity to Sars-CoV-2 works but also help inform future vaccine strategies.
They said 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.
Prof Moss said: “To our knowledge, our study is the first in the world to show robust cellular immunity remains at six months after infection in individuals who experienced either mild/moderate or asymptomatic Covid-19.
“Interestingly, we found that cellular immunity is stronger at this time point in those people who had symptomatic infection compared with asymptomatic cases.”
Reflecting on the implications for natural immunity after infection, he said: “I think this data is reassuring, potentially even encouraging, but it does not mean that people cannot get re-infected.
“We now need large-scale population studies to show how the antibody and cellular profiles act together to protect people over time.
“This cannot be taken as confirmation of an immunity passport. Absolutely cannot do that.”
Professor Charles Bangham, chair of Immunology, Imperial College London, said: “This excellent study provides strong evidence that T-cell immunity to Sars-CoV-2 may last longer than antibody immunity.
“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.”