Leading global health scientist Devi Sridhar believes it's "'very unrealistic" that the Tokyo Olympics will take place in July next year unless a coronavirus vaccine is discovered.
The global COVID-19 pandemic, which has killed more than 150,000 globally, saw the Games pushed back a year and is now scheduled to take place from July 23 to August 8, 2021.
But Dr Sridhar, chair of global health at the University of Edinburgh, said the chances of Tokyo going ahead as planned "all depends on a vaccine".
Her comments come after International Olympic Committee (ICO) coordination commission chair John Coates admitted COVID-19 could still affect the rescheduled Olympics.
As it stands, there is no 'plan B' if the IOC needs to postpone the Games again.
"We're hearing from the scientists that this could be possible. I had thought it would be a year or a year-and-a-half away, but we're hearing possibly this could come sooner," Dr Sridhar told the BBC.
"If we do get a vaccine within the next year then actually I think that [Olympics] is realistic. The vaccine will be the game-changer - an effective, affordable, available vaccine.
"If we don't get a scientific breakthrough then I think that looks very unrealistic.
"I think they've made the right decision in saying 'we are going to put it back a year and re-evaluate'.
"And I think that is the only way you can deal with this situation - to take stock, to be hopeful, to support our scientific community and our NHS community to do what they can, because science in the long-term is going to be the way we get out of this."
In Japan, there is a state of emergency because of the country's worsening coronavirus outbreak.