TOKYO (BLOOMBERG) - The number of people seriously ill with Covid-19 in Tokyo could surge in the coming weeks, peaking as the Olympics are under way, even without thousands of participants streaming into the capital.

A new analysis shows severe coronavirus cases could rise to a level that would require another state of emergency by early August in Tokyo, despite progress in vaccinating the elderly - if current restrictions in Japan's urban areas are lifted as scheduled on June 20.

The disease modelling from Kyoto University professor Hiroshi Nishiura was presented to government officials at a coronavirus advisory board meeting on Wednesday (June 9).

With the Olympics set to start in under two months, many people have focused on the risk posed by tens of thousands of overseas athletes and support staff arriving in Japan, which has been effectively closed to visitors since the beginning of the pandemic.

Experts now are zeroing in on domestic factors that could contribute to an uptick in cases that would coincide with the games.

University of Tokyo public health researcher Haruka Sakamoto said that there are four consecutive holidays right before the Olympics, as well as summer vacation and Obon holiday, when people traditionally travel home to visit the graves of their ancestors.

She said: "It's easy to imagine that more and more people will think: 'If the Olympics can be held, it's okay for us to travel'."

That may cause an increase in the number of people infected, Ms Sakamoto added.

International Olympic Committee vice-president John Coates said last month that the games will be held even if Tokyo is under a state of emergency.

Japan has seen upticks in cases after previous emergency restrictions were eased.

When the first such measure ended in May last year, cases spiked at the end of July.

Epidemiologists also pointed to the fact that people were likely to retreat indoors to air-conditioned rooms in the height of summer, creating conditions where the virus spreads more easily.

It would be difficult to determine if an increase in cases would be due to the Olympics or other factors.

But Ms Sakamoto said that a jump in infections would strain the medical system, especially if they occurred in younger people - many of whom do not yet qualify for vaccination in Japan and are more likely to seek critical care if their symptoms turn serious.

The current state of emergency in parts of Japan has led to some of the most severe policies to date, such as asking restaurants to not serve alcohol.

The number of new infections has dropped, with Tokyo's seven day moving average of new cases falling by about half in the last month. It is not clear what restrictions will remain in place when the emergency status is lifted.

Japan's vaccination program, which got off to a slow start, has picked up speed in recent weeks. More than 20 million doses have now been administered in the country of 126 million people.

Currently, enough shots have been given to cover 7.7 per cent of the population, according to Bloomberg's vaccine tracker.

Japan's vaccine coverage is still the lowest among the world's most developed nations.

Prof Nishiura's model estimates that the prevalence of severe cases will be much lower than they would have been if no vaccinations were being given. Still, Tokyo's critical care capacity would not have enough beds.

The model does not take into account vaccination rates among those younger than 65, a group that is expected to gain more access to vaccination in the coming weeks.