* The study — published in Nature on July 29 — says that in the blood of a person previously exposed to common cold, blood cells called T cells remember the viral attack

* Those who have antibodies against SARS-CoV-2 are generally considered immune to Covid-19

Who would have thought a blocked or runny nose would one day be a cause for celebration? But a study by a group of scientists states that some blood cells in a human body recall a previous episode of the common cold — and the memory prompts them to block the novel coronavirus, or the Covid-19 virus.

The study — published in Nature on July 29 — says that in the blood of a person previously exposed to common cold, blood cells called T cells remember the viral attack. Subsequently, when that person is exposed to the novel coronavirus (SARS-CoV-2), which causes Covid-19, these T cells — based on their memory of the common cold attack — come to the rescue. They react with the spike protein of SARS-CoV-2 and block the virus, thereby conferring immunity to the person.

Prior infections with coronaviruses that cause the endemic “common cold” might induce immune cells or T cells, Leif Erik Sander of Charité – Universitätsmedizin Berlin, Germany, and one of the authors of the study, tells BL ink .

In the study, researchers from several German institutes such as the Charité and the Max Planck Institute for Molecular Genetics, looked at immune cells collected from the blood samples of 18 Covid-19 patients undergoing treatment (at Charité), and 68 healthy individuals who did not have any previous exposure to SARS-CoV-2. T cell-mediated immunity was observed in 24 (35 per cent) of the healthy individuals who had not been infected and thus did not have antibodies against the virus, and 15 (83 per cent) of the Covid-19 patients.

“If pre-existing T cell immunity to SARS-CoV-2 can mediate a kind of protection for unexposed people, [it] could help to understand the epidemic. More importantly, it has to be assessed in detail in vaccination studies,” adds Andreas Thiel, another author of the study and a professor at the same institute.

Those who have antibodies against SARS-CoV-2 are generally considered immune to Covid-19. Herd or community immunity refers to a situation where over 60 per cent of people in a community develop these antibodies. This means every member of the community — even the 40 per cent who were not exposed to the virus — are now immune to it.

To assess antibody levels against SARS-CoV-2, serological surveys are being carried out in Indian states, yielding impressive results. For instance, 57 per cent of slum dwellers in Mumbai and nearly 23.48 per cent of Delhi’s population have shown antibodies against the Covid-19 virus. The Mumbai study was carried out in July by the Brihanmumbai Municipal Corporation in collaboration with Niti Aayog and the Tata Institute of Fundamental Research, while in Delhi it was conducted from June 27 to July 10 by the National Centre for Disease Control (NCDC) in collaboration with Delhi government.

Some experts hold that in addition to the antibody-mediated immunity, the immunity provided by T cells against SARS-CoV-2 can further enhance herd immunity in India.

“As data becomes available from vaccine trials over the next few months, we will appreciate if T cells are sufficient to protect us,” points out Zania Stamataki, a viral immunologist at the University of Birmingham in the UK.

Some experts call for more verification. Gregory Poland, director of US-based Mayo Clinic’s Vaccine Research Group and editor-in-chief of the renowned journal Vaccine , stresses the need to verify the data more widely.

“It is possible that such cross-reacting antibody could offer at least some partial protection against Covid-19 (this would need to be verified), and thus ‘raise’, in part, herd immunity, but again this needs a lot more verification,” he points out.

According to Diptendra Sarkar, professor of surgery and Covid-19 strategist affiliated to the Kolkata-based Institute of Postgraduate Medical Education and Research, most serological surveys looking for antibodies against SARS-CoV-2 rely on a type of antibody called immunoglobulin G (IgG). But the IgG rise in patients’ sera is temporary, he stresses.

“T cell surveys can go a long extent in predicting the long-term herd immunity of a community,” he adds. Danny Altmann, a professor of immunology at the Imperial College, London, also holds that antibodies against SARS-CoV-2 wane within a few months. “T cell immunity, on the other hand, is easily detectable for decades,” he adds.

The study calls for a wider survey in India. Sarkar stresses that the government and health governing agencies need to look at the issue of cell-mediated immunity.

But while the T cell is being studied, what is perhaps more important now is vaccine development and implementation. According to David Corry, professor of pathology and immunology and medicine at Baylor College of Medicine, US, even with a 35 per cent rate of T cell reactivity against SARS-CoV-2 — as suggested in the Nature paper — the virus still causes widespread illness and death.

“You must have antibodies [that a vaccine can develop in the human body], thus you must be vaccinated,” Corry tells BL ink . Stamataki agrees. “Vaccination has the potential to elicit potent, long-lasting immune responses, and this is necessary for herd immunity,” she adds.

Sanjeet Bagcchi is a physician and independent writer based in Kolkata

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