“The ideal is always to strive for 100 per cent of vaccination rate, but this may not be possible for practical reasons,” Lim told the Post. “For example, people with health conditions that may not be suitable for vaccination, people who do not consent to the vaccination, and differences in availability of vaccines among countries taking part in the Olympic Games.

“In most public health settings, 70 to 80 per cent of a vaccination rate would be accepted as the level that achieves herd immunity.”

Around 11,000 athletes and 7,000 officials are expected to arrive for the Tokyo Games, which open on July 23 and runs until August 8. IOC President Thomas Bach said this month that 85 per cent of athletes and officials living in the Olympic Village would be fully vaccinated, rising from a June prediction of 75 per cent.

The IOC also said in March, when vaccines were not widely available, that 270 world championships and world cups held between September 2020 and March this year resulted in no outbreaks, despite involving more than 30,000 athletes.

It is not mandatory for participants to be vaccinated but Olympic advisers have said it hardly mattered because vaccination had never been the primary strategy against an outbreak in the first place. Medical and sports analysts have said the 85 per cent rate offered an acceptable coverage considering the herd immunity threshold of 75 to 80 per cent set by most countries.

Measures to prevent the spread of the virus – with the Olympics deemed a potential superspreader event by some doctors ­– remain paramount.

“Eighty-five per cent is what we’re working with. It is good enough because that’s what we have. It is better than 84 per cent. It will substantially reduce the risk even more,” said Brian McCloskey, a public health adviser to the IOC who chairs an independent expert panel that developed Covid-19 countermeasures for the Games.

“I would doubt whether you could calculate a statistically significant difference between the effect of 85 per cent and 90 per cent. It just wouldn’t make a huge difference. When [the IOC] started planning the kind of measures that we would use for the Games, we did not consider vaccination as part of that. So vaccination is a bonus on top of these measures.”

All Olympic participants will observe a strict regimen of testing, masking and social distancing as outlined in the IOC’s “playbook” that provides safety guidelines for the Games. They must undergo daily coronavirus screening and will be sent to the Athletes Village’s fever clinic if they have a temperature of 37.5 degrees Celsius or higher or test positive. In Japan, their locations must remain traceable by GPS and they should stay within the village, competition venue or training grounds. No cheering, hugging and high-fives are permitted.

Winners will not have medals placed around their necks but must take them off a tray during the presentation ceremony. Organisers have threatened to fine or disqualify rule-breaking athletes, and even expel them from Japan.

While officials have come under fire for insisting on staging the Games amid a pandemic, other big events have been held this year such as grand slam tennis tournaments the French Open and Wimbledon.

The European Championship football tournament that ended in July had spectators returning to stadiums at reduced capacities. Fans were asked to mask up and keep a distance of at least 1.5 metres, while certain stadiums required proof of a negative coronavirus test. For the final, fans were asked to show proof of full vaccination or a negative coronavirus test with London’s Wembley Stadium allowing 60,000 spectators in to watch England lose on penalties to Italy.

Still, social-distancing measures were ignored as maskless supporters gathered after the match. The WHO warned that Euro 2020 crowds could become superspreader events and authorities in England, Scotland and Denmark reported an uptick in coronavirus cases because of those games.

British Prime Minister Boris Johnson defended the decision of allowing more than 60,000 spectators to attend the final, as the vaccines had created “a considerable wall of immunity”.

Yet for the Olympic Games, organisers had always planned for a “bubble”. Striving for a 100 per cent vaccination rate minimises the risks but is not possible for multiple reasons, said Brett Toresdahl, a sports medicine doctor at New York’s Hospital for Special Surgery.

“A vaccination rate of over 80 per cent combined with the other prevention measures have lowered the risk of Covid-19 to a level that is acceptable to the vast majority of athletes and their medical teams,” Toresdahl said.

“The estimated vaccination rate of over 80 per cent within the Olympic and Paralympic Village is currently higher than in any other country,” he added.

Two out of five Afghan athletes were inoculated with the Pfizer-BioNTech vaccine, under the IOC’s agreement with the companies, in the Qatar vaccination hub, its National Olympic Committee told the Post.

Tracking down the vaccination rates of each nation’s delegations is also an uphill task. The IOC declined to reveal the number of vaccines provided to each country. Most Olympic committees did not respond to the Post’s requests for comment. The few that did decline to reveal the number of athletes who have already received jabs.

In Hong Kong, close to 100 per cent of its 46 athletes competing in the Olympics will be vaccinated. Yet, the exact figure remains undisclosed, deemed by the Hong Kong Sports Institute as “very personal and related to athletes’ privacy”.

The US Olympic and Paralympic Committee is also not tracking or mandating its 800-plus athletes to be vaccinated but replies from responsive Olympic committees offer a glimpse into the difficulties of getting athletes and staff inoculated.

Slovakia has around 90 per cent of its 41 athletes vaccinated, its Olympic committee said. “Just a few members were not interested in getting vaccinated. Some were infected by Sars-CoV-2 in February or March, so they were still within the 180 days period of natural immunisation,” its spokesman said, using the coronavirus’ scientific name.

As for Malawi, one out of its five taking part athletes is underage. The 15-year-old will attend the Games unvaccinated. The delegation was inoculated with the AstraZeneca jab under its country’s vaccine roll-out, and the WHO has not recommended it for those aged under 18.

Despite the risks, the IOC is absolving itself of any responsibility should an athlete contract the virus. All athletes must sign a waiver – said to be typical for major sports events – but with wording that releases organisers from any Covid-19 liability.

Certain sports could also be at a greater risk of Covid-19 infection, especially close-contact indoor sports such as fencing, boxing and wrestling, according to Lisa Brosseau, a respiratory-protection research consultant for the University of Minnesota’s Centre for Infectious Disease Research and Policy.

Any sport that takes place outdoors with no close contact, such as archery, golf and surfing, are at low risk. Current safety measures also do not recognise the exposure to human-generated aerosols, or small particles, in indoor spaces, said Brosseau. In shared spaces and in proximity to others, Brosseau warned the current safety measures are not effective.

“Masks are not effective at limiting the emission or inhalation of small infectious particles generated during breathing or talking,” said Brosseau. “I believe transport will occur in small buses. I am concerned about the bus driver who will have the highest exposure – meeting many people in a day.”

Furthermore, not all staff, volunteers and members of the media would be fully vaccinated during the Games, said Toresdahl. The IOC said 70 to 80 per cent of the media representatives would be vaccinated while all 70,000 volunteers were expected to be inoculated.

“While the available vaccines are very effective, Covid-19 infections can still occur,” said Toresdahl.

Lim from Singapore said extra precautions athletes must take at the Games may also lower their protection levels. “Another risk is the impact that travelling, time zone differences and competition stress have on the immune system. That may lower the resistance to counter the effects of the Covid-19 virus,” said Lim, though he said there was insufficient data to fully understand its risks.

McCloskey, the independent health adviser to the IOC, however, said there was little evidence to suggest the coronavirus could be spread at a sporting event. “We had positive cases at the tennis and football tournaments, but those happened from people being infected at home, not during the field of play,” he said.

Even before vaccines were widely available, many indoor contact sporting events were held with minimal Covid-19 transmission when testing and prevention measures were implemented, said Toresdahl.

“As the pandemic continues and sporting events resume, event organisers and medical staff are learning more about how to minimise the risk of Covid-19 to an acceptable level,” said Toresdahl.