An experimental drug has, in a breakthrough, shown to significantly reduce fatigue in people suffering from long COVID, according to results of a phase-II clinical trial.

Severe fatigue is the most prevalent symptom faced by long COVID patients. So far, there are no treatment options available for long COVID conditions, which also include anxiety, breathlessness, among others.

The trial, reported in the journal eClinicalMedicine, showed that participants given the drug AXA1125, developed by US pharmaceutical company Axcella Therapeutics, reported feeling less fatigued than those given a placebo.

AXA1125 was tested for long COVID fatigue as previous data from Axcella showed effects on cellular energetics and inflammation.

"The reduction in patients' own reports of fatigue is really positive news, and we hope that further work will help us understand the underlying processes behind this improvement too," said Betty Raman, Associate Professor from Radcliffe Department of Medicine at the University of Oxford in the UK, who conducted the research.

"There is still some way to go in treating all patients with long COVID -- our results focus specifically on fatigue, rather than the breathlessness and cardiovascular issues that other long COVID patients have reported," Raman added.

Of the 41 patients taking part in this study, half had the investigational treatment (an orange-flavoured powder dissolved in water) twice daily for four weeks, while the other half had a placebo.

On average, patients had symptoms of fatigue for about 18 months prior to entering the study. All the patients who started the study completed it, and none reported serious adverse effects of either the treatment or the placebo.

Emerging data on long COVID suggests that the virus targets the mitochondrial, which are essential to normal energy generation and control of inflammation. AXA1125 may improve energy generation and reduce the amount of inflammation in the body.

As a result, the team also tracked mitochondrial health in the patients' muscles before and after they took the medication, using magnetic resonance spectroscopy scans of the patients' calf muscles as they bent and straightened their leg against the mild resistance from an exercise band.

The scans showed no overall difference between the patients who took the treatment versus the placebo. But, those in the treatment arm did report significantly improved fatigue levels.

Those who reported an improvement in fatigue also had improved mitochondrial health and walked further compared to those without.

The research team hopes that future studies will determine if the treatment is effective for an even larger group of long COVID patients.