The antiviral, molnupiravir, does not reduce hospital admissions or deaths from coronavirus in high-risk vaccinees, new research has found.
But the treatment was associated with a shorter recovery time of four days and a reduced viral load.
People who received molnupiravir reported feeling better compared to those who received usual care, the study found.
The researchers suggest that although the drug may have some benefit in terms of reducing symptoms, the cost of the drug may mean that it is not the best choice for the general population, given the study results.
But it can be useful in reducing pressure on UK health systems, they added.
The drug is one of the most expensive antivirals used to treat Covid, with a seven-day course costing around US$700, or £577.
Molnupiravir (brand name Lagevrio) was the first treatment to be studied by the adaptive trial platform of new antivirals for the early treatment of Covid-19 in the community (Panoramic).
The study was set up to identify high-risk groups of people who were most likely to benefit from new antiviral treatments for the virus.
Chris Butler, professor of primary care at 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 big frontier in our research response to the ongoing global pandemic.
“It is in the community that treatments could have massive reach and impact.
“But decisions about who to treat should always be based on evidence from rigorous clinical trials involving people for whom the drugs would most likely be prescribed.”
He added: “We must not forget the other ongoing antibiotic resistance pandemic, which stems in part from the large-scale use of antimicrobial drugs before we do rigorous clinical trials to find out who really benefits. treatment and who does not.”
But Prof Butler said there could be circumstances in which the decision could be made to use the treatment.
He explained: “Although this trial found no benefit of molnupiravir treatment over its primary outcome – which assumed that treatment with molnupiravir for at-risk vaccinated patients would reduce the likelihood of hospitalization or death – the trial suggests that this treatment may have other benefits when used to treat Covid-19, such as faster recovery time and reduced follow-up with health services.
“It could help ease the burden on UK health services by treating selected patients at home, during times of high disease burden and pressure on key services.
“We therefore hope that this new evidence will be useful to policymakers when preparing strategies for managing Covid-19 infections during the winter.”
People enrolled in the study were within five days of symptom onset and either healthy over the age of 50 or aged 18 to 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 NHS standard of care.
Explaining the results, Professor Judith Breuer, Director of the UCL Pathogen Genomics Unit, said: “Results generated by the UCL substudy show that participants receiving molnupiravir cleared SARS-CoV-2 more quickly than those who only received usual care.
“As part of Panoramic, we are continuing to study the impact of the drug on the virus genome (genetic material) and on the antibody responses of participants.”
Professor Sir Jonathan Van-Tam, Pro-Vice-Chancellor of the Faculty of Medicine and Health Sciences at the University of Nottingham and co-author of the study, said: “While molnupiravir has originally found to be effective in reducing hospitalization of patients with Covid, these were unvaccinated patients.
“This latest research has repeated the exercise in the highly vaccinated population, demonstrating that vaccine protection is so strong that there is no clear benefit of the drug in terms of further reduction in hospitalizations and deaths.
“However, the duration of symptoms and virus shedding are both markedly reduced, and we have to wait much longer to find out if there will be any noticeable effects on long Covid.”
The results are published in the journal The Lancet.