Omicron: You may forget the COVID-19 variant but here's why your T-cells won't

The view that the immune system — prepped via either recovery from infection, or vaccination — can tackle the new variant has found support in a South African study that credits T-cells with putting up a robust defence against Omicron

Kenneth Kumar Mohanty December 31, 2021 11:03:25 IST
Omicron: You may forget the COVID-19 variant but here's why your T-cells won't

Representational image. PTI

Countries are going into fresh curfews and restrictions are coming back again amid fresh surges under the shadow of the Omicron variant of the novel coronavirus. Public authorities everywhere are worried about health systems buckling under a load of severe cases, something that the likes of India especially can ill afford after the second wave in the middle of 2021 showed the kind of devastation that can occur if cases shoot up all of a sudden.

But some have sought comfort in the theory that Omicron, even if more transmissible, causes a milder infection than the Delta variant. The view that the immune system — prepped via either recovery from infection, or vaccination — can tackle the new variant has found support in a South African study that credits T-cells with putting up a robust defence against Omicron. But what are T cells?

What Has The Study Found?

Researchers in South Africa, which was the first to report the emergence of a new variant with an unprecedented number of mutations at the end of November 2021, said they have found that T cells — a part of the immune system — are able to successfully take on the Omicron variant.

“Despite Omicron’s extensive mutations and reduced susceptibility to neutralising antibodies, the majority of T cell response, induced by vaccination or natural infection, cross-recognises the variant," the researchers say in a paper that is yet to be peer-reviewed. Cross-recognition, or cross-reactivity, is the phenomenon where protection tailored to target a specific antigen is successful in beating back another as well.

Reports said that the researchers conducted test tube experiments in which they exposed copies of the virus to T cells from volunteers who had received either the Johnson & Johnson single-dose vaccine, the two-dose regimen of the Pfizer-BioNTech jab or had not been vaccinated but recovered from a bout of Covid-19 infection with an earlier version of the novel coronavirus.

They found that even though Omicron’s mutations might be helping it to escape antibodies, which are described as being the body’s first line of defence against infection, the newest variant of concern (VoC) cannot escape the body’s second-line defence, the T cells. The researchers concluded that T cells were are highly effective at recognising and attacking the Omicron variant, thus preventing most infections from progressing to critical illness.

“Well-preserved T cell immunity to Omicron is likely to contribute to protection from severe Covid-19," they said.

What kind of defences does the immune system have?

While it might appear that the novel coronavirus has made it a habit to defy all kinds of evasive or safety measures, it turns out that there are, in fact, two layers of defences the immune system can present to a pathogen like the novel coronavirus or Sars-CoV-2.

The so-called first line of defence, says the World Health Organisation (WHO), is the body’s ‘innate immunity’, which is the “general immediate response to any infection".

Move past that and the virus encounters 'adaptive immunity', which is the “specific response to an infection" built via cellular response — the T cells — and the antibody response, which involves what are known as memory B cells.

If the innate immunity kicks instantly upon brush with a pathogen, the cellular and antibody response “usually starts after six to eight days", WHO says.

For people who have been vaccinated or have recovered from infection, their T and B cells, upon a subsequent encounter with the same virus, “respond rapidly and the immune system can effectively clear an infection before it causes disease". WHO adds that vaccines use this immune memory to protect us from infection.

The European Centre for Disease Prevention and Control (ECDC), an agency of the European Union (EU), explains that after a bout of infection or vaccination, “it is the adaptive immune response that ideally delivers long-term protection". The memory B cells “produce different classes of antibody to neutralise the virus or virus-infected cells", while the memory T cells “support antibody production and also have a direct role in killing virus-infected cells".

So, what are T-cells?

The ‘T’ in T cells stands for thymus, which is the organ in which the cells’ final stage of development occurs. T cells are basically white blood cells that devise different means to counter specific diseases.

Experts at the Imperial College London said that while more research is needed “to fully understand the relationship between our T cells and immunity to Covid-19… early findings suggest that they may provide us with long-term protection against the disease".

“T cell responses could also help to explain why some people recover relatively quickly from Covid-19, but others continue to suffer chronic after-effects for months following infection," said Rosemary Boyton and Danny Altmann, both professors at the Imperial College London. They explain that T cells can play different roles, act either as “killer cells" that attack cells which have been infected with a virus, or as “helper cells" that support B cells to produce antibodies.

Noting that with other coronavirus infections like Sars and Mers, the assumption has been that T cell responses “offer far more durable protection", the duo said in August 2020 — after they had published a paper on T cell immunity and Covid-19 — that the “the good news is that most people who’ve been infected, from those who are hospitalised through to those who are asymptomatic, seem to have decent levels of T cell immunity".

Where do T-cells score?

Profs Boyton and Altmann said that T cells are able to mount “a very widely targeted immunity", that is, they can target “an array of different proteins made by the virus".

But while T cell responses can be long-lasting and more powerful, “the only catch is that researchers haven’t proved that T cells in their own right are protective". Further, while it is relatively simple to conduct as serological test to determine if the body has built up antibodies against a virus following infection or vaccination, “methods for T cell testing are reasonably high-tech hospital laboratory tests".

Antibodies, the first line of defence, are geared towards targeting the first line of attack of the virus, which in the case of the novel coronavirus is its spike protein, the spear-like structures studding its surface that it uses to invade and latch on to human cells.

The Imperial College experts said that “antibodies probably wane quite quickly, and it might only be possible to detect them eight to ten weeks after infection", which means that “you no longer have any immediate protection from the virus after that time".

Citing a study, ECDC says that “neutralising antibodies… are detectable within seven to 15 days of disease onset, and levels increase until days 14-22, before plateauing and then decreasing" but memory B cells “persisted for over 242 days post-symptom onset".

“Similarly, the development of memory T cells directed at non-surface Sars-CoV-2 proteins following infection or vaccination may offer a route to durable immunity where virus evolution leads to spike protein mutations that escape pre-existing neutralising antibodies," it adds.

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