There is "clear-cut" evidence that a drug can help people recover from the coronavirus, say US officials.
Remdesivir cut the duration of symptoms from 15 days down to 11 in clinical trial at hospitals around the world.
The full details have not been published, but experts said it would be a "fantastic result" if confirmed, but not a "magic bullet" for the disease.
A drug would have the potential to save lives, ease pressure on hospitals and allow parts of lockdown to be lifted.
Remdesivir was originally developed as an Ebola treatment. It is an antiviral and works by attacking an enzyme that a virus needs in order to replicate inside our cells.
The trial was run by the US National Institute of Allergy and Infectious Diseases (NIAID) and 1,063 people took part. Some patients were given the drug while others received a placebo (dummy) treatment.
Dr Anthony Fauci who runs the NIAID said: "The data shows remdesivir has a clear-cut, significant, positive effect in diminishing the time to recovery."
He said the results prove "a drug can block this virus" and were "opening the door to the fact that we now have the capability of treating" patients.
The impact on deaths is not as clear cut. The mortality rate was 8% in people given remdesivir and 11.6% in those given a placebo, but this result was not statistically significant, meaning scientists cannot tell if the difference is real.
It is also not clear who is benefiting. Is it allowing people who would have recovered anyway to do so more quickly? Or is it preventing people from needing treatment in intensive care? Did the drug work better in younger or older people? Or those with or without other diseases? Do patients have to be treated early when the virus is thought to peak in the body?
These will be important questions when the full details are eventually published, as a drug could have the twin benefit of saving lives and helping to lift lockdown.
Prof Mahesh Parmar, the director of the MRC Clinical Trials Unit at UCL, who has overseen the trial in the EU, said: "Before this drug can be made more widely available, a number of things need to happen: the data and results need to be reviewed by the regulators to assess whether the drug can be licensed and then they need assessment by the relevant health authorities in various countries.
"While this is happening we will obtain more and longer term data from this trial, and other ones, on whether the drug also prevents deaths from Covid-19."
If a medicine can stop people needing intensive care then the risk of hospitals being overwhelmed is smaller, and there is less need for social distancing.
Prof Peter Horby, from the University of Oxford, is running the world's largest trial of Covid-19 drugs. He said: "We need to see the full results, but if confirmed this would be a fantastic result and great news for the fight against Covid-19.
"The next steps are to get the full data out and work on equitable access to remdesivir."
The US data on remdesivir has come out at the same time as a trial of the same drug in China, reported in the Lancet medical journal, showed it was ineffective.
However, that trial was incomplete because the success of lockdown in Wuhan meant doctors ran out of patients.
"These data are promising, and given that we have no proven treatments yet for Covid, it may well lead to fast-track approval of remdesivir for treatment of Covid," said Prof Babak Javid, a consultant in infectious diseases at Cambridge University Hospitals.
"However, it also shows that remdesivir is not a magic bullet in this context: the overall benefit in survival was 30%."
Other drugs being investigated for Covid-19 include those for malaria and HIV which can attack the virus as well as compounds that can calm the immune system.
It is thought the anti-virals may be more effective in the early stages, and the immune drugs later in the disease.
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