DUBAI: People who recovered following a moderate or an asymptomatic coronavirus episode could have T-cell immunity for as long as six months. This was borne out of a study that showed patients have kept T-cell immunity against SARS-CoV-2 for up to 180 days after primary infection.
The level of immunity was found even higher in symptomatic patients, a new study shows. Data for the research, led by Dr. Jianmin Zuo of the University of Birmingham's Institute of Immunology and Immunotherapy, came following an evaluation of 2,000 people, including 100 non-hospitalised healthcare workers in March and April. SARS-CoV-2-specific T-cell responses to SARS-CoV-2 were detected in the 100 healthcare workers.
The research found a robust T-cell-driven immunity has been maintained at 6 months following primary infection. The study, published as a pre-print in Biorxiv on November 2, 2020, had not been peer-reviewed. If results are confirmed by expert reviewers, the confirmation of the longevity of the T-cell immune response to SARS-CoV-2 would be critical — both in controlling primary infection and preventing re-infection, researchers pointed out.
Importantly, the study is the first peek into data on the level of T-cells, the so-called "killer" cells of the immune system — antibodies that have the ability burn away pathogens (disease-causing agents like SARS-CoV-2). Moreover, the study shows immunity remains in the body six months after primary infection in people even with mild (asymptomatic) symptoms. This is noteworthy as statistically, majority of infections are mild/asymptomatic, researchers pointed out.
Previous studies suggested that following primary infection, antibody levels can decline within the first few months. One potential implication: As of end-August, about 6,000 of the 100,000 persons (6%) whose blood samples taken in the UK were found with SARS-CoV-2-specific antibodies — proteins produced in response to the virus. The antibody levels, however, dropped by more than a quarter in three months, researchers revealed in late October.
If the virus somehow hurdles the first line of defence, the immune system buys time until the “experts” show up. These experts constitute two types of white blood cells: B-cells and T-cells. They work in tandem. If the virus makes it past the initial assault, some T-cells become "killers", burning respiratory cells invaded by the pathogen (SARS-CoV-2 virus).
Helper T-cells, on the other hand, take on a supportive role. One role it takes is by stimulating B-cells to produce even more antibodies that immobilise the virus enough to stop them from penetrating other cells. The adaptive system also retains memory — but in the case of COVID-19, it was unclear how much and for how long.
Robust cellular immunity
Having a robust cellular immunity for at least for six months after even mild or asymptomatic SARS-CoV-2 infection, is good news.
“This data is reassuring,” lead study author Paul Moss, from the University of Birmingham, told a Science Media Centre briefing. As in any other study, further research is recommended to show whether or not such level of immunity can fight off re-infection, or how long the protection lasts. "We need to have much larger population studies to show that,” Moss told journalists.
No immunity passport
The findings can’t be taken as confirmation that an "immunity passport" would be feasible,” Moss pointed out. The study is believed to be the first in the world to show that a robust cellular memory against the virus persists for at least for six months.
The donors had an average age of 41 (range 22 to 65 years old); 23 were men and 77 were women. None of them were hospitalised with COVID-19 — 56 people had mild or moderate symptoms and 44 were asymptomatic.
Serum samples were collected monthly to measure antibody levels and blood samples were taken after six months to measure the T-cell response. The study found that virus-specific T cells were detectable in all donors at six months (via ELISPOT and ICS analysis).
Antibody levels fell by around 50% during the first two months after infection but then plateaued. The magnitude of the T cell response at six months was strongly correlated with the magnitude of the peak antibody response, the study found.
The fact that data pointed to 50% higher T cell response in those who had experienced symptoms did not necessarily mean that asymptomatic people may be more susceptible to reinfection, Moss said, as they may just be better at fighting off the virus without the need to generate a large immune response.
The findings have implications for vaccine development too. The cellular response was directed against a range of proteins from the virus, including the "S" (spike) protein currently targetted by most COVID-19 vaccine research. As T cell responses were also directed against additional nucleoprotein and membrane proteins, these could also be valuable targets for future vaccines strategies, the researchers noted.
“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,” Fiona Watt, executive chair of the Medical Research Council, told the British media.
“This excellent study provides strong evidence that T cell immunity to SARS-CoV-2 may last longer than antibody immunity,” added Charles Bangham, chair of immunology at Imperial College London. “These results provide reassurance that, although the titre of antibody to SARS-CoV-2 can fall below detectable levels within a few months of infection, a degree of immunity to the virus may be maintained. However, the critical question remains: do these persistent T cells provide efficient protection against re-infection?”