ew hopes were raised today that the body has a “second line of defence” against Covid-19 that could help to prevent reinfections.
UK scientists, in what is believed to be a world first, found that people who contracted coronavirus during the first wave of the pandemic all developed a cellular or T-cell response to fighting the infection.
Today’s preliminary findings were described as “encouraging” and come after experts last week revealed that antibodies – the body’s other natural defence mechanism – appeared to wane after several months.
The study, based on samples provided by Public Health England staff, also offer encouragement in the search for a vaccine as they indicate that T-cells can be activated by the presence of the virus.
However it is not yet known to what degree the T-cells protect against re-infection, though the early indications are that they may last longer than antibodies.
T-cells go hunting in the body for other cells containing the virus to kill them off. In previous pandemics such as SARS, they have been known to remain in the body for a decade.
Today’s study, by PHE and UK Coronavirus Immunology Consortium, which has not yet been peer reviewed, found “robust” T-cell responses to covid six months after infection.
It is thought that the sicker the person was with covid, the stronger the cellular response.
However, the study only included people of working age – meaning that it is currently unknown to what degree older people, who are most at risk of covid, generate T-cells.
The 2,000 volunteers in the study included 100 people who were found to have developed antibodies after mild to moderate symptoms or no symptoms at all. T-cell responses were found in all 100 people.
The size of T-cell response differed between individuals, being 50 per cent higher in people who had symptoms.
It is possible that heightened cellular immunity might provide increased protection against reinfection in people with symptoms.
Alternatively, those without symptoms may have been able to fight off the virus without the need to generate a large immune response.
Professor Paul Moss, UK Coronavirus Immunology Consortium lead from University of Birmingham, said: “Understanding what constitutes effective immunity to [Covid 19] is extremely important, both to allow us to understand how susceptible individuals are to reinfection and to help us develop more effective vaccines.
“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. We now need more research to find out if symptomatic individuals are better protected against reinfection in the future.”
Dr Shamez Ladhani, consultant epidemiologist at PHE and the study’s author, said: “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.”
He added: “The idea you can’t get re-infected is not true. Reinfections do occur, but we would expect that a tiny minority of people will get re-infected.”
Professor Fiona Watt, executive chair of the Medical Research Council, said: “This is promising news – if natural infection with the virus can elicit a robust T-cell response then this may mean that a vaccine could do the same.”
Professor Charles Bangham, chair of immunology, Imperial College London, who was not involved in the research, 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, the critical question remains: do these persistent T-cells provide efficient protection against re-infection?”
He added: “It will also be important to follow the antibody and T-cell immunity in people who develop the syndrome of Long Covid – the persistent and sometimes debilitating condition that follows acute SARS-CoV-2 infection in a still uncertain proportion of people.”