Long COVID or Long-haul COVID is a group of health problems persisting or developing after an initial period of COVID-19 infection.

Symptoms can last weeks, months or years and are often debilitating.

The Guardian has estimated that two million people in the UK are experiencing Long COVID, and global figures have shown that around 400 million people worldwide suffer from the condition.

Key symptoms include fatigue, brain fog and muscle aches and pains, although at least 200 symptoms have been documented.

Professor Luke O’Neill told the Show Me The Science podcast that around 59% of people with Long COVID will have some definite signs of organ damage.

“So, their lungs might be scarred or there might be evidence of blood vessel damage, these things are relatively common in a lot of people with it,” he said.

“But of course, researchers are searching for the root cause, it’s not clear what causes it.

“However, people are sort of grouping around a few things to say these might be a cause; one is clotting.

“That began to emerge early actually, my lab worked on this aspect of COVID, we published a big paper on the clotting aspect of COVID.

“So, the COVID virus will cause your blood to clot and there’s some evidence of aberrant clotting in certain parts of your body.

“Things called micro clots, tiny clots form, and that might affect that tissue and damage it.”

Wuhan Coronavirus COVID-19 2019-nCoV microbiology lab test. Chemist with protective uniform & gloves holds vacutainer blood tube with vaccine trial. Image: bestravelvideo / Alamy. 30 January 2020

Prof O’Neill also said cytokines – key molecules that control the immune system – seem to play a part in Long COVID.

“We know certain cytokines cause fatigue and they cause what’s called sickness behaviour during infection, so that seems to be part of it,” he said.

“The third one is histamine... There is evidence that some people are overreacting to their own histamine or they’re making too much histamine.

“Now, histamine is made usually in an allergy, and you know hay fever, the symptoms of that are often caused by histamine."

Economic impact

The effects of Long COVID have extended far past the individuals suffering from the illness, as Prof O’Neill said that many of those affected never return to work.

“There was one study in the UK that one-in-five doctors came down with Long COVID and then less than a third of them got back to working full time,” he said.

“Now, that’s important because, you know, what does Long COVID do to people? It means they can’t work, or they’ve got to work part time.

“That has an effect on them, obviously, their mental health - and also their economic earning potential.

“Then on society as a whole, we can’t have lots of skilled people out of work or not being able to work fully.”

Prof O’Neill said that there are trials being conducted at the moment which have prescribed patients with antihistamines and anti-inflammatories to try and manage their Long COVID.

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Main image: Luke O'Neill in the Newstalk studio (L) and an image of a COVID-19 blood test (R).