Dwindling T cells might also be to blame for why the elderly are much more severely affected by Covid-19.  

Hayday points to an experiment conducted in 2011, which involved exposing mice to a version of the virus that causes Sars. Previous research had shown that the virus – which is also a coronavirus and a close relative of Covid-19 – triggered the production of T cells, which were responsible for clearing the infection.

The follow-up study produced similar results, but the twist was that this time the mice were allowed to grow old. As they did so, their T cell responses became significantly weaker.

However, in the same experiment, the scientists also exposed mice to a flu virus. And in contrast to those infected with Covid-19, these mice managed to hold onto their T cells that acted against influenza well into their twilight years.

“It’s an attractive observation, in the sense that it could explain why older individuals are more susceptible to Covid-19,” says Hayday. “When you reach your 30s, you begin to really shrink your thymus [a gland located behind your sternum and between your lungs, which plays an important role in the development of immune cells] and your daily production of T cells is massively diminished.”

What does this mean for long-term immunity?

“With the original Sars virus [which emerged in 2002], people went back to patients and definitely found evidence for T cells some years after they these individuals were infected,” says Hayday. “This is again consistent with the idea that these individuals carried protective T cells, long after they had recovered.”

The fact that coronaviruses can lead to lasting T cells is what recently inspired scientists to check old blood samples taken from people between 2015 and 2018, to see if they would contain any that can recognise Covid-19. The fact that this was indeed the case has led to suggestions that their immune systems learnt to recognise it after being encountering cold viruses with the similar surface proteins in the past.   

This raises the tantalising possibility that the reason some people experience more severe infections is that they haven’t got these hoards of T cells which can already recognise the virus. “I think it’s fair to say that the jury is still out,” says Hayday.

Unfortunately, no one has ever verified if people make T cells against any of the coronaviruses that give rise to the common cold. “To get funding to study this would have required a pretty Herculean effort,” says Hayday. Research into the common cold fell out of fashion in the 1980s, after the field stagnated and scientists began to move to other projects, such as studying HIV. Making progress since then has proved tricky, because the illness can be caused by any one of hundreds of viral strains – and many of them have the ability to evolve rapidly.   

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