T-CELL immunity to the coronavirus could last up to six months – but does not mean people cannot be reinfected, experts have claimed.
Researchers found that immunity to the killer bug is present in most adults six months after they were first infected.
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The study, which has not yet been peer-reviewed, was formed of research from Public Health England (PHE) and the UK Coronavirus Immunology Consortium (UK-CIC).
The findings demonstrated robust T-cell responses to Covid-19 peptides at six months in all participants following asymptomatic, mild or moderate infection.
A T-cell is one of the main adaptive components of the immune system, its role in the body is to kill infected host cells and activate other immune cells.
Professor Paul Moss, UK-CIC lead and professor of haematology at the University of Birmingham, said the new data was “reassuring, potentially even encouraging”.
One expert added that while the data was “thorough”, it could not tell us whether or not people can become reinfected.
Prof Brendan Wren, Professor of Microbial Pathogenesis, London School of Hygiene & Tropical Medicine added: “A significant and prolonged cellular immune response is observed in both symptomatic and asymptomatic infected individuals.
“If cellular immunity is a good correlate of protection to re-infection, this provides encouraging data for long-term immunity to SARS-Cov-2 and the development of effective vaccines.”
As part of the research, experts collected serum and blood samples from more than 2,000 clinical and non-clinical healthcare workers.
This was made up of 100 people, 77 women and 23 men, who had tested positive for Covid after a blood test.
This is referred to as sero-positive and these people were between the ages of 22 and 65.
All of the 100 individuals who tested sero-positive had 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.
Various tests were then carried out to assess different aspects of the T-cell response.
These included the magnitude of response and the response to different proteins from Sars-CoV-2.
The authors said that this is more complicated than antibody studies and added that this study is currently the largest in this field to date.
Analysing the samples over the last six months and the experts found the T-cell responses were present in all individuals.
The immune response was directed against a range of proteins from the virus and this includes the current Spike protein which is being used in several vaccine studies across the world.
Immunity was also present in 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.
Experts found that the size of T-cell response differed between individuals, being considerably higher (50 per cent) in people who had experienced symptomatic disease at the time of infection six months previously.
While the experts say more research needs to be done, they added that the 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.
They added that cellular immune responses needed to be elicited in vaccine regimes, due to the fact that the antibodies waned over time.
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.”
He added that large-scale population studies are needed to show how the antibody and cellular profiles can act together and protect people over time.
“This cannot be taken as confirmation of an immunity passport. Absolutely cannot do that”, he added.
Dr Shamez Ladhani, consultant epidemiologist at Public Health England and the study’s author, said: “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.
“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.”
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.”
However, one expert claimed that it is not correct to claim that this is evidence of T-cell immunity.
Prof Eleanor Riley, Professor of Immunology and Infectious Disease, University of Edinburgh said immunity is a functional term that implies resistance to infection, and this she says, has not been directly shown in this study.
“Determining whether these T cell responses are protective against reinfection would require either an experimental infection study (such studies are under active discussion) or a very large, long term study to assess the frequency of reinfection in people whose T cell responses have been measured.
“Given the apparently very low incidence of reinfections at the current time, such a study would not currently be a high priority.
“But if our worst fears are realised, and reinfection turns out to be a significant problem, then such studies would help to identify “correlates of protection” – immunological markers of protection versus susceptibility to reinfection”, she added.