The antiviral, molnupiravir, does not reduce coronavirus hospital admissions or deaths in vaccinated people at high risk, new research suggests.
But the treatment was associated with a shorter recovery time, by four days, and reduced viral load.
People who received molnupiravir reported feeling better compared to those who received usual care, the study found.
Researchers suggest that while the drug could have some benefits in terms of symptom reduction, the cost of the drug may mean it is not the best choice for the general population, given the study findings.
But it may be useful in reducing the pressure on UK health systems, they added.
The drug is one of the more expensive antivirals used to treat Covid, with a seven-day course costing around 700 US dollars, equivalent to £577.
Molnupiravir (brand name Lagevrio) was the first treatment to be studied by the platform adaptive trial of novel antivirals for early treatment of Covid-19 in the community (Panoramic).
The study was set up to identify which groups of higher-risk people were most likely to benefit from new antiviral treatments for the virus.
Chris Butler, professor of primary care in the Nuffield Department of Primary Care Health Sciences and co-chief investigator of Panoramic, said: “Finding effective, safe and scalable early treatments for Covid-19 in the community is the next major frontier in our research response to the ongoing worldwide pandemic.
“It is in the community where treatments could have a massive reach and impact.
“But decisions about who to treat should always be based on evidence from rigorous clinical trials that involve people who would most likely be prescribed the drugs.”
He added: “We must not forget the other ongoing pandemic of antibiotic resistance, which in part stems from using antimicrobial drugs at scale before we did rigorous clinical trials to find out who really benefits from treatment, and who does not.”
But Prof Butler said there may be some circumstances where the decision might be made to use the treatment.
He explained: “Although this trial found no benefit from molnupiravir treatment on its primary outcome – which hypothesised that treatment with molnupiravir for vaccinated, at-risk patients would reduce the likelihood of hospitalisation or death – the trial suggests that this treatment could have other benefits when being used to treat Covid-19, such as a faster recovery time and reduced follow-up with health services.
“This could help to ease the burden on UK health services through the treatment of selected patients at home, during times of high disease burden and pressure on key services.
“We therefore hope this new evidence will be of use to policymakers when preparing strategies for managing Covid-19 infections over the winter.”
Those enrolled in the study were within five days of symptoms beginning and either aged over 50 years in good health or between 18-50 with underlying health conditions that made them clinically more vulnerable.
A total of 25,786 people were randomly assigned to receive either molnupiravir or the usual standard of NHS care.
Explaining the findings, Professor Judith Breuer, director of the UCL Pathogen Genomics Unit, said: “The results generated by the UCL sub-study show that those participants receiving molnupiravir cleared SARS-CoV-2 more quickly than those receiving only usual care.
“As part of Panoramic, we are continuing to investigate the impact of the drug on the virus’ genome (genetic material) and on the participants’ antibody responses.”
Professor Sir Jonathan Van-Tam, pro-vice-chancellor for the Faculty of Medicine and Health Sciences at the University of Nottingham and study co-author, said: “While molnupiravir was originally found to work well to reduce hospitalisation in patients with Covid, these were unvaccinated patients.
“This latest research has repeated the exercise in the highly vaccinated population, demonstrating that the vaccine protection is so strong that there is no obvious benefit from the drug in terms of further reducing hospitalisation and deaths.
“However, symptom duration and virus shedding are both markedly reduced, and we have to wait much longer to know if there will be any discernible effects on long Covid.”
The findings are published in The Lancet journal.