By James Gallagher
Inside Health presenter, BBC Radio 4
The initial emergency of Covid may feel like it's over, but thousands of people are still suffering from ongoing symptoms known as long Covid. So are we any closer to understanding the causes of this debilitating condition or to finding the best way of treating it?
Neil Robinson is finding it hard to accept that he is still affected by Covid. "I was just so certain that I would have recovered by now," he told me.
Jo House is learning to adjust to her new reality. "It feels weird to say it, but I'm now disabled and I need to rethink how I live my life," she says.
And although John Dusabe Richards is now improving, he has not fully recovered. A year after a mild Covid infection he could not read a bedtime book to his children because of the "sandbag on my chest" that made it hard to breathe. The breathlessness, constant headaches and joint pain are largely gone, but his fatigue is lingering. Dancing or doing sports with his kids needs planning due to an unnaturally long recovery time.
"Every time I think that I'm close to recovery, I push too far and then relapse and it is bad. I have to remain within the limits of what I can do," he says.
This pattern will be familiar to many with long Covid. The core experience of long Covid revolves around the unholy trinity of fatigue, brain fog and muscle pain. But there is a long list of rarer symptoms covering almost every bodily function ranging from digestive problems to night sweats and depression to hair loss.
The Office for National Statistics says one in 40 people have symptoms that last at least three months after an infection. Around 1.5 million people in the UK say they have long Covid symptoms with growing numbers (685,000) saying their long Covid has lasted longer than a year.
"This is a picture that we're seeing more and more," Dr David Strain, from the NHS long Covid taskforce and the University of Exeter, told me.
It seems remarkable that a viral infection, even one that was brief and mild, can lead to debilitating symptoms that, for some people, can last for years.
There is no single unifying cause of all long Covid, but scientists are focusing on four main areas:
Micro-clots: Some symptoms of long Covid could be down to tiny clots blocking up the smallest blood vessels in our bodies, called capillaries. Our major arteries and veins are like motorways up and down our body, but capillaries do the actual job of getting oxygen and nutrients to the individual cells of our bodies. Crucially, capillaries also clear away the waste products of our metabolism. Blocked capillaries could lead to cells becoming rapidly distressed.
"That would account for why some people start the day feeling great because the body's had time to replenish itself, but very, very quickly, they become exhausted," says Dr Strain.
It could also account for fatigue that lasts for days after strenuous activity as "those toxins build up and they're just not clearing at the normal rate," he says.
Research has also shown the brain can shrink by between 0.2% and 2% even after mild infection so unrepaired damage is a potential cause of symptoms like brain fog.
Immune system: Scientists have also reported changes in the immune system of people with long Covid. Inflammation is part of the body's normal response to infection or injury, but proteins in the blood suggest excessive inflammation in long Covid which could be driving some symptoms.
Meanwhile, antibodies that normally attack foreign invaders have been shown to stick to the body's own cells in long Covid. Autoantibodies are antibodies that mistakenly target and react with a person's own tissues or organs. These could cause different symptoms depending on where they bind, explains Prof Chris Brightling, from the University of Leicester: "Those with muscle pain and weakness might have an autoimmune problem related to muscles, those with an affected heart rhythm might have autoantibodies to the heart tissue."
Persistent infection: There is also debate about whether the virus is fully kicked out of the body in some with long Covid. The infection starts in the lungs and airways, but the virus is able to infect other parts of the body too. The idea is that the virus takes up residence elsewhere in the body, most likely the in gut which is already teeming with microbial life.
"It is just leaking enough virus into the blood that every so often you are back to the beginning again, you are experiencing that flu-like illness and you are exhausted for no apparent reason," says Dr Strain.
Mitochondrial damage: Another concept centres around the body's ability to harness energy, possibly due to abnormalities in the mitochondria. Mitochondria, which are in nearly every cell of our body, are like minute power stations that convert the energy from food into a form usable by the body.
"There are people suggesting that the virus itself makes those mitochondria go dormant, which is why we run out of energy much quicker in the fatigue and in the brain fog," says Dr Strain.
Long Covid clinics have been set up, but there are no proven cures for the condition.
Neil says his "biggest problem" is fatigue and that when he developed brain fog and confusion it reminded him of when his grandfather first got dementia, as the symptoms were "almost identical".
Neil started going to a clinic a year ago and was given breathing exercises and his activity and energy levels were monitored. But recent physical and mental tests found that he was "no better", which leaves him in limbo. "I can't be discharged, but they don't currently have anything else to offer," he says.
But understanding the range of possible causes of long Covid is giving new ideas for therapies.
High levels of inflammation mean there is a good argument that antihistamines may become very important, says Prof Brightling, as the drugs could calm the immune response down in the same way they do in allergies.
Blood-thinning drugs that make it harder for clots to form are being investigated and even drugs that boost mitochondria function are being trialled.
Meanwhile, if long Covid is caused by a persistent infection it raises the prospect of antivirals, vaccines or lab-made antibodies being useful.
Prof Brightling says healthier lifestyles may help some people as the severity of long Covid has been linked to obesity and other health conditions.
Dr Strain says some people need to be careful about trying to exercise their way out of long Covid. He says it has worked for some, but many will find it makes their condition worse. "It takes much longer to recover from exercise than the benefit you get… it's one step forward, two steps back."
He recommends finding the brain and body's limit before exhaustion sets in and then going no further than 80% of that limit before taking a rest. "For some people that could be walking a mile, for others it could just be standing up for 10 minutes to peel the carrots," he says.
However, that is easier said than done. Jo House has gone from working full time to just one day and even that is spread over the week. The usual family structure has been flipped on its head with her "amazing boys" taking on the role of carer.
"Every once in a while I want to try and have some fun and do something normal like socialise with my friends or take the dog for a walk," she says.
"What happens is I collapse back into bed. It feels like I've run a marathon and I've ended up in A&E a couple of times. So I think actually my problem is I am not good enough yet at pacing and resting."
It is impossible to know what the long term prognosis of people with long Covid is, however, there are strong parallels with another condition – chronic fatigue syndrome (CFS) also known as myalgic encephalomyelitis (ME). The cause has not been proven, although infections are a leading candidate, and it has been notoriously difficult to treat.
Dr Strain says "in all likelihood, ME and long Covid are the same family of conditions with a different underlying trigger" and many with ME/CFS "are left with symptoms from that for years, if not decades afterwards".
Prof Brightling warns that although society is moving on from the pandemic, there are still large numbers of coronavirus infections and there is still a need to understand the long term implications of being infected and how to treat long Covid.
"There is a real risk of a pretty awful legacy, there is a real chance to get it right, but also a real chance to get it very wrong," he says.
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By James Gallagher