An antiviral drug tested by more than 25,000 vaccinated Covid patients has been found to reduce recovery time from the disease.
Molnupiravir was given to people twice a day, for five days at home, while they had the Omicron variant of Covid.
Despite aiding recovery, the drug did not decrease death rates or hospital admissions.
Participants taking the antiviral drug while having Covid were compared with those receiving standard care who also had the infection.
The trial was conducted to see whether it backed up previous studies on molnupiravir, which had suggested it was effective at reducing hospital admissions among patients with mild-to-moderate Covid.
However, those trials were conducted on unvaccinated patients, before the emergence of the Omicron wave.
Research in this latest study showed the treatment reduced recovery time by around four days and also reduced the viral load - the level of infection.
Molnupiravir, which is made by Merck, Sharp and Dohme (MSD), is very expensive- a seven-day course costs around £577.
It was the first antiviral drug studied as a treatment for Covid in the community - which means it was taken at home rather than in a medical setting.
Chris Butler, who is a professor of primary care in the Nuffield Department of Primary Care Health Sciences, says: "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."
To participate in the trial, people had to be within five days of the start of symptoms and either healthy and over 50 or 18-50 with underlying health conditions that made them more vulnerable to Covid.
Former deputy chief medical officer for England, Prof Sir Jonathan Van-Tam, who is 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.