A COVID antiviral developed by Japanese researchers can shorten the symptoms as well as the number of days one can test positive post-infection by the SARS-CoV-2 virus, according to the trial data.

The oral antiviral drug ensitrelvir was co-developed by Hokkaido University and pharma company Shionogi. The trial tested its efficacy to accelerate recovery in about 1,200 people, Nature reported.

The oral 125-milligram ensitrelvir pills helped clear a stuffy or runny nose, sore throat, cough, feeling hot or feverish and low energy or tiredness about a day earlier, compared with those in the control group.

Those who took the pill also tested negative for COVID about 29 hours earlier than did those who took a placebo.

The results were presented at the Conference on Retroviruses and Opportunistic Infections (CROI) in Washington. It has not been peer-reviewed yet.

Further, according to the drug's manufacturer Shionogi, the trial also showed that ensitrelvir has the potential to prevent long COVID. People who took the antiviral had a 14 per cent risk of developing long COVID. Those who did not take the drug had a 26 per cent risk of developing long COVID conditions.

No drug has been conclusively shown to reduce the risk of long COVID. Only preliminary evidence hints that Pfizer's Paxlovid might have this effect.

Scientists, however, are sceptical about Shionogi's claim and are critical of the design of the clinical trial of ensitrelvir, Nature reported. They point out that the trial was not specifically intended to investigate the risk of long COVID.

It's unclear whether Shionogi's definition of long COVID was determined before the trial began, physician Eric Topol, director of the Scripps Research Translational Institute in San Diego, California was quoted as saying. Because this was an exploratory phase of the study, it's not possible to make any strong conclusions, he added.

According to the head of clinical development at Shionogi in Florham Park, New Jersey, the company could not specify the plan for analysing long COVID data ahead of time because long COVID was less clearly defined in the past than now.

He says that these results, although not definitive, will shape an ongoing trial evaluating ensitrelvira�s effect on COVID-19 symptoms, Nature reported.