SINCE Scotland's first known Covid infection was identified in a patient from Tayside on March 1 2020, at least 1.37 million positive cases have been confirmed - equivalent to a quarter of the whole population.

The actual number of Scots exposed to the virus will be far higher, however, given the low availability of tests at the beginning of the pandemic and the large percentage of cases which have been asymptomatic - especially following vaccination.

According to estimates based on confirmed cases, Office for National Statistics (ONS) surveillance, and modelling from Cambridge University, two thirds of England's population are believed to have been infected at least once.

Scotland would not be far behind.

One of the most puzzling legacies of the pandemic has been the so-called 'long haulers' - those whose infections appear to have morphed into a debilitating and chronic illness characterised by symptoms ranging from fatigue and brain fog to pain, breathlessness, and even rashes.

Gauging the precise scale of the problem is difficult, however.

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The ONS provides estimates based on self-reported incidence, where Long Covid is defined as symptoms persisting beyond four weeks which cannot be explained by anything else.

Their latest figures for Scotland put the number of Long Covid sufferers at 100,000 as of January 2, with 40,000 of these individuals - equivalent to around 1-3% of those infected to date - having lived with their symptoms for more than 12 months.

Nearly a quarter (24%) said their daily lives had been "affected a lot" by the condition, compared to 18% in England.

HeraldScotland: The cumulative of confirmed Covid cases in Scotland is close to 1.4m, equivalent to a quarter of the population having been infectedThe cumulative of confirmed Covid cases in Scotland is close to 1.4m, equivalent to a quarter of the population having been infected

The challenges of dealing with Long Covid are manifold: there is no diagnostic test, no treatment, and no scientific consensus on what causes it - or even what it is.

"Our basic view is that there is no such thing as 'Long Covid'," said Professor Alan Carson, a consultant neuropsychiatrist and researcher at Edinburgh University's Centre for Clinical Brain Sciences.

"To be clear, I'm not suggesting for one second that there are not a lot of patients who complain of very real and long-lasting symptoms after Covid.

"But I take the view that it's a mistake to think of Long Covid as a single entity. I think it's a group of conditions."

This is important, said Prof Carson, because different forms of the illness will probably require different treatments.

He is currently leading a study which is trying to unravel the underlying causes behind the common cognitive ailments which plague Long Covid sufferers, such as memory problems and difficulty concentrating.

This includes taking detailed MRI scans of patients' brains and analysing blood and cerebrospinal fluid for proteins known to be released when the brain is harmed.

In some cases - particularly in older patients where Covid infections led to delirium - the virus appears to have accelerated the onset of dementias, such as Alzheimer's disease, which typically take many years to develop.

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In other patients, cognitive impairment seems to be more closely linked to fatigue brought about by damage to lung tissue which has weakened the respiratory systems.

It is unclear, however, whether Long Covid is genuinely more common, or worse, than other post-viral illnesses.

"My suspicion is that Covid is not that unique," said Prof Carson.

"What is perhaps unique about it for our lifetimes is the sheer number of people who got it all in one go.

"But it's quite hard to get definitive figures and rates [for Long Covid], and it's also quite hard to get definitive rates for comparator illnesses."

HeraldScotland: Professor Alan CarsonProfessor Alan Carson

He added: "It's not always been well researched but there were several very well-conducted studies in the 1990s in Australia which found that recovery from viral illness is quite slow for a lot of people.

"So it's maybe not so much that Covid is worse, as that viruses are a bit worse in terms of their impact on human functioning than we tend to associate them.

"There certainly seems to be huge similarity between post-viral symptoms associated with other viruses and symptoms associated with Covid.

"My own feeling is that Covid probably isn't any worse, but there's a lot of uncertainty."

Jane-Claire Judson, chief executive at Chest Heart & Stroke Scotland, said more progress is needed on support services for Long Covid sufferers.

She said: “We’re two years into the pandemic and tens of thousands of people in Scotland are living with the debilitating effects of Long Covid and many families are still suffering in silence.

“People who were fit and healthy are telling us that they’re struggling to get out of bed and they fear they’ll never be able to return to full-time work or support their families.”

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Meanwhile, recent weeks have seen a flurry of research findings related to Long Covid.

Data from a rapid review by the UK Health Security Agency indicated that fully vaccinated people who contract Covid are less likely to go on to experience Long Covid symptoms.

The protective effect appeared to be strongest in the over-60s.

In January, research led by Dr Emily Fraser, a consultant at Oxford university hospitals, was also the first in the world to demonstrate that microscopic abnormalities in lung tissue can be detected in patients suffering from long-term breathlessness after Covid.

The study, involving 36 patients, used specialised MRI imaging in which patients breathe in xenon gas while lying in a scanner.

It found “significantly impaired gas transfer” from the lungs to the bloodstream, something impossible to detect using a standard CT scan.

The study is ongoing and is aiming to recruit 400 participants in total.

“It does suggest the virus is causing some kind of persistent abnormality within the microstructure of the lungs or in the pulmonary vasculature,” said Dr Fraser.

Other studies have highlighted the way Covid infections seem to increase the risk of everything from cardiovascular complications to mental health disorders long after recovery.

HeraldScotland: MRI scanners have been used to study Long Covid patients in more detailMRI scanners have been used to study Long Covid patients in more detail

An analysis of 150,000 US veterans found that their risk of experiencing a stroke increased by 52% during the year following the infection, while their risk of heart failure went up by 72%.

The authors, writing in Nature, cautioned that since the medical records were dominated by white, male adults the findings may not apply universally.

Nonetheless, they said the results were striking.

“I am actually surprised by these findings that cardiovascular complications of Covid can last so long,” said Hossein Ardehali, a cardiologist in Chicago and co-author of the paper.

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A separate study, also based on US veterans but published in the BMJ, found an increased incidence of depression - at a rate of around 15 extra cases for every 1000 people - among those who had recovered from Covid.

They were also more likely to experience sleeping problems, abuse drugs and alcohol, and had a 50% higher rate of suicidal thoughts compared to those who had never tested positive for Covid. Dr Paul Poulakos, a New York psychiatrist who commented on the research, noted that protective measures put in place during the pandemic to reduce social interaction and curtail the virus may have exacerbated the problem.

He said: “The disease is unique from a mental health standpoint because many of the recommendations that have been put in place to protect people from Covid-19 are, in fact, risk factors for mental illness.”