This suggests that what we’re calling “long Covid” is probably not a single, isolated syndrome. Instead, it’s an umbrella term that we using to describe a mixture of post-viral manifestations united by a prior diagnosis of coronavirus infection.

The reported rates may also be misleading. Much of the data is based on self-reported symptoms and voluntary surveys. Both are highly vulnerable to recall bias: people with a recent brush with Covid may be more motivated to participate in studies and to remember details that others dismiss or are irrelevant. In essence, some end up attaching significance to a coincidence. And someone with asymptomatic coronavirus infection, for instance, which might account for up to half of cases, is unlikely to have been tested and hence won’t report that they are feeling absolutely fine in the aftermath.

Some studies have tried to address this, like Terence Stephenson’s analysis from UCL of thousands of teenagers, comparing symptoms reported by those with positive and negative Covid tests. This enables a more pure signal of long Covid’s features to be extracted from the noisy symptomatic hubbub of other winter infections and the emotional distress of being in a pandemic situation. These results suggested that up to one in seven of the kids in the study with a history of coronavirus infection did show some features of long Covid.

Tellingly, though, presumably the children who were negative for Covid were being tested because they were nevertheless symptomatic owing to infection with some other circulating viruses, like colds. And the fact that a reasonable proportion of them had some symptoms similar to long Covid reminds us that the majority of viral infections often produce a post-viral syndrome. Lethargy, poor sleep, weight changes, aching joints and poor concentration are common symptoms, as anyone diagnosed with glandular fever knows only too well. This probably reflects changes in the activity of the immune system and metabolism in the immediate aftermath of fighting off the infection.

At the moment, the jury is out and scientific opinion is divided. Some speculate that, in some people, the virus hangs on in the aftermath of acute infection, retreating to some reservoir site in the body where, beyond the reach of the immune system, it provokes a chronic low-grade reaction. This, they argue, accounts for the energy-sapping symptoms described by more than half of long Covid sufferers. In support of this theory, coronavirus particles have been detected in the intestines of some patients, many months after recovery. 

Other authorities point to the fact that women are more frequently affected by long Covid, drawing parallels with the fact that they are also more susceptible to so-called “autoimmune” diseases. This is where the immune system goes rogue and certain body tissues fall victim to friendly fire. 

This autoimmune hypothesis seems credible, because many viruses have track records of causing this to happen. Type 1 diabetes, thyroid disease and some conditions resembling multiple sclerosis have all been linked to prior viral infections. It’s likely that, because viruses grow inside our own cells, in learning to fight off the virus the immune system is occasionally and erroneously coaxed into regarding some of our own cells as hostile and the attack widens to healthy tissue too.

At the moment, we don’t know which hypothesis is the correct one. Nor do we know how best to manage the condition. An inflammatory disease might benefit from suppressing the immune system for a while. But if persistent infection turns out to be the driver, this would be like pouring petrol on a bonfire. Some individuals have also reported improvements when they’ve been Covid vaccinated, although others have reported the opposite, perhaps for the same reason.

The problem is further compounded by the fact that, although scientists are beginning to pick up signature changes in the body’s biochemistry of long Covid victims that are absent in unaffected cases, there’s no reliable test yet for long Covid. This means that for the 1.3 million – roughly one in 50 – people that the Office for National Statistics (ONS) estimate currently have long Covid, getting an accurate diagnosis remains a challenge. Another unappetising prospect is the potential for claims of “long Covid” to become the occupational health virological equivalent of a whiplash for the work-shy, making genuine cases of long Covid harder for victims to defend. Let’s hope not.