T-cell immunity and COVID-19
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Immunity observed even without detectable antibodies highlights the need to study T-cell response
- Our immune system responds to virus infections with a first-line defence called ‘innate’ immunity, followed by the second-line called ‘adaptive’ immunity.
- Innate immunity is like first aid — an immediate response, not strong enough to prevent pathology if the virus is highly virulent or the ‘inoculum’(infecting virus load) is heavy.
- Innate immunity then passes the baton to adaptive immunity, which takes several days to develop and become effective.
- Adaptive immunity has two arms — antibodies and T-cell immunity. Antibodies are protein molecules that recognise and bind to viral antigens.
- Some among them tend to neutralise viruses from infecting fresh host cells. Some viruses then adopt other mechanisms to infect host cells, and that is when T-cell immunity may come to the rescue.
- In most viral infections, the presence of antibodies in the blood is sufficient to classify individuals as immune.
- In persons with asymptomatic infections or mild COVID-19, nearly half will have no detectable antibodies after two months.
- This phenomenon of short-lived antibodies and consequent re-infection is also seen in some other respiratory tract viruses. Generally, re-infections are mild or asymptomatic, presumably due to protection afforded by T-cell immunity.
- Knowing that reinfection with symptoms has so far been proven in only about ten cases among millions infected, protective immunity after the first infection is probably durable. The observed protection in the face of non-detectable antibodies highlights the need to study T-cell immunity.
- In COVID-19 infection, T-cell immunity is more long-lasting than antibodies. It resides in a subset of white blood cells called T-lymphocytes, or T cells.
- However, the test for assessing T-cell immunity is complicated and expensive. Researchers from CardiffUniversity have come up with a simplified and rapid T-cell immunity test, called ‘T- SPOT test’, that can be done in many laboratories.
- Serial evaluation of T-cell immunity can help determine its durability after vaccination. Therefore, it is no surprise that vaccine trials have started testing for T-cell immunity too.
- The immune T cells had ‘stem-cell’ like characteristics — indicating their long-term survival and potential of quick multiplication. A study from Birmingham confirmed that in COVID-19, T-cell immunity is durable and lasts for more than six months.
- Four coronaviruses causing common cold are widely prevalent in human communities.
- Two of them are Beta-coronaviruses, the phylogenetic group to which the COVID-19 coronavirus belongs. The prevalence of cross-reacting T-cell immunity from the common cold coronaviruses is likely to vary from country to country, depending partly on population density and the frequency of recurrent viral infections of the respiratory tract.
- Countries with high population densities, where such infections spread quickly, may be expected to have a higher proportion of the population exposed to them. This may explain the relatively lower impact of COVID-19 (in terms of number of cases and deaths per million population) in countries like India and many low-income countries.
Obviously, T-cell immunity is a better and more durable marker than antibodies of past infection for this novel virus. If India’s vaccination policy, when made, recommends that vaccines may be conserved for priority use for non-immune subjects, then, a rapid T-cell immunity test, such as the one developed in Cardiff, will be better than antibody tests. Therefore, developing simple and rapid assays for T-cell immunity should be a priority for Indian scientists to work on, quickly. Those with T-cell immunity may need no vaccine, or only a single dose of a two-dose vaccine regimen.