Thousands of Olympic volunteers and officials began receiving vaccines in Tokyo on Friday, five weeks before the Games, as experts warned it would be safest to hold the event without fans.
Organisers said they would meet on Monday to discuss how many domestic spectators to allow, if any, with reports suggesting they would opt for a cap of 10,000 people.
With just over a month until the July 23 opening ceremony, organisers are in the home stretch and scrambling to finalise virus rules, and get participants vaccinated in time.
Japanese Olympic athletes have already begun receiving vaccines, and the rollout expanded on Friday to Olympic staff, volunteers and others who will interact with overseas participants.
The International Olympic Committee has donated enough Pfizer/BioNTech doses for 40,000 people, including airport staff, local media and Olympic referees.
The jabs are separate from those being used in the national vaccine rollout, which began slowly but has picked up pace lately -- with over six percent of the population now fully inoculated.
"Now that I will be vaccinated, I will feel a little more reassured doing my job," Chika Hirai, director of doping control for Tokyo 2020, told reporters as she got her jab.
The vaccinations come as organisers work to convince a sceptical public that the biggest international event since the pandemic began will be safe.
This week they have rolled out new virus rulebooks, warning athletes they could be barred from the Games if they violate regulations on mask-wearing or daily testing.
Fans from abroad are already banned from attending the postponed event -- but organisers still face a controversial and difficult decision over how many domestic fans, if any, will be in the stands.
"Holding the Games without spectators is a better way to control the risk and to realise a safe and secure Games," Tokyo 2020 chief Seiko Hashimoto told reporters.
But "as long as we have spectators who would endeavour to see the Games on site, we would like to accommodate their desire as much as possible, and we should do whatever we can to minimise the risks".
Her comments followed discussions with a group of leading medical experts who advise the government and said Friday that a closed-door Games would be safest.
They urged Olympic organisers to "impose stricter standards" of infection control if they allow fans, also recommending limits on spectators from outside the area.
Hashimoto said that if the virus situation is "still very challenging" in Japan when the Games are held, organisers would be prepared to reverse course and ban fans.
"We may be obliged to make the very difficult decision of not welcoming any spectators at the very last minute," she cautioned.
The number of fans at the Games will be limited by government virus measures, which in Tokyo currently cap spectators at 5,000 people or 50 percent capacity, whichever is smaller.
That rule is scheduled to stay in place until July 11, even though a virus state of emergency will end on Sunday.
After July 11, the cap will be raised to 10,000 people or 50 percent capacity, and Japanese media reports said a similar cap was likely to be announced by Games organisers.
Tokyo 2020 said Friday they have further slashed the number of overseas participants coming to Japan for the Olympics and Paralympics to 53,000, not including around 15,500 athletes.
That is down from original plans for 177,000 people, including officials, sponsors and media, they said.
Tokyo 2020 also said Friday they had received offers from more than 100 overseas volunteer medical staff.
The foreign volunteers facilitated by the IOC are meant to help ensure the Games do not place extra pressure on Japan's medical system.