The Tokyo Olympics next July will be a "uniquely risky" event, demanding flexibility from organisers amid the uncertainty of the coronavirus pandemic, particularly if a vaccine has not been rolled out by then, medical experts say.
Japan and the International Olympic Committee made the unprecedented decision last month to delay the Games for a year, as the world battles the virus that has infected 2.3 million people and killed more than 150,000 globally.
But questions persist whether the Games can go ahead 15 months from now, as a vaccine could still be at least a year away, according to the most optimistic estimates.
"When we talk about bringing sports back with packed stadiums, I really think that is something we are going to have to wait for a vaccine to be able to do," said Zach Binney, an epidemiologist at Emory University in the United States.
Tokyo 2020 is set to run from July 23 to August 8 next year, but organisers expect few changes to the original plan, including attendance by enthusiastic supporters.
But that might be over optimistic, Binney, a specialist in aspects of athlete health, told Reuters.
"Every person that you add to a gathering adds risk," said Binney, who expects vaccine development would take 1-1/2 years from the start of the outbreak, or until late 2021, at the earliest.
"So once you get up to 50,000, 70,000, 100,000 (fans)... that is an enormous amount of risk to be taking on without a vaccine."
The Olympics was "a uniquely risky event", he added, because of the threats represented by visitors streaming in from areas with a lot of infections, and the reverse flow when they return afterwards, perhaps carrying home infections.
Although more hopeful for a vaccine to be found within a year, Jason Kindrachuk, an infectious diseases expert at the University of Manitoba in Canada, flagged a risk of further delay to the Games, as vaccinating people would take time.
"It is going to push the limits right up to the border of when the Olympics should be starting," said Kindrachuk, who has worked on outbreaks of Ebola and SARS.
"You want to get people vaccinated not right at the point of the Olympics, but a little bit beforehand, so they build up that protective immunity."
Professor Kentaro Iwata, who drew international attention for his criticism of the Japanese government after a visit in February to the Diamond Princess cruise ship quarantined with thousands of passengers, expressed pessimism over the new date.
"I don't think the Olympics is likely to be held next year, if held at any time," the virologist told the Foreign Correspondents Club of Japan in a teleconference.
"Japan might be able to control this disease by next summer, and I wish we could, but I don't think that would happen everywhere on earth," the Kobe University professor added.
In response, the Tokyo 2020 Organising Committee said it was focusing purely on delivering the Games next year.
"The mission is to prepare the stage for next summer; we do not feel it is appropriate to respond to speculative questions," spokesman Masa Takaya said in an email to Reuters on Monday.
Even if a vaccine were not ready, the organisers might still be able to hold the Games, as long as they were flexible and well prepared, Kindrachuk added.
"It is going to take some creative thinking, and hopefully some willingness from the public, to at least still partake in watching... understanding that it is just simply not going to look like what we have seen before."
Binney said the safest option would be for athletes to arrive in Tokyo four to six weeks in advance and follow rigorous isolation measures for two weeks before moving into the Olympic village to train for the remaining period.
"My goal would be to have the Olympic Village as a virus-free quarantine zone," he added.