A blood test to diagnose long Covid is on the horizon after scientists discovered part of the immune system continues to be active long after the infection has passed.

Researchers at Cambridge University found an immunity molecule is still present in sufferers from the condition in a discovery that suggests the symptoms may be as a result of the immune system not shutting off properly.

Figures from the Office for National Statistics (ONS) this month show that nearly one million people in Britain report ongoing problems at least four months after Covid, with 40 per cent still suffering a year after an infection.

The most common symptoms include fatigue, shortness of breath, muscle ache and difficulty concentrating. Such symptoms are also common in people suffering viral infections and are often a result of the immune response rather than the infection itself. 

Scientists now believe long Covid may be caused by the immune system failing to dial down after the body has successfully fought off an infection.

In a pilot study of 85 sufferers, researchers found that, alongside memory T-cells and antibodies showing a previous infection, there was another immune molecule – or cytokine – present.The cytokine could act as a way to diagnose the condition as well as helping scientists to understand what causes it.

Dr Nyarie Sithole, from the Department of Medicine at the University of Cambridge, who co-led the team said: "One of the theories of what’s driving long Covid is that it’s a hyperactive immune response – in other words, the immune system switches on at the initial infection and for some reason never switches off or never goes back to the baseline. 

"As we’ll be following our patients for many months post-infection, we hope to better understand whether this is indeed the case.

"Because we currently have no reliable way of diagnosing long Covid, the uncertainty can cause added stress to people who are experiencing potential symptoms. If we can say to them ‘yes, you have a biomarker and so you have long Covid’, we believe this will help allay some of their fears and anxieties."

The team also wants to find out whether patients with long Covid improve after being vaccinated. There has been anecdotal evidence that symptoms lessen after a jab, and researchers can now look for the molecule to find out whether there has been a change. 

"Our study will allow us to see how this biomarker changes over a longer period of time in response to vaccination," added Dr Sithole.

The team is now increasing the study size to 500 patients and has received funding from the National Institute for Health Research to develop a test.