Japan's top COVID-19 adviser Shigeru Omi said Thursday that the peak of the fifth coronavirus wave has largely passed, but warned that a close eye must still be kept on the country's overburdened medical system.

Speaking to the House of Councillors' health committee, Omi attributed the downtrend in infections to a "combination of factors," namely the progress of vaccinations, reduced foot traffic, suspension of alcohol in dining establishments and high rate of mask-wearing.

Shigeru Omi, chairman of a government subcommittee on the coronavirus response, speaks during a House of Councillors committee session in Tokyo on Sept. 16, 2021. (Kyodo)

But he cautioned that hastily easing anti-pandemic restrictions on people's lives could lead to a "sixth wave" of infections, especially with colder weather approaching.

"We should be aware that the number of hospital beds (for COVID-19 patients) will not increase five to six-fold all at once," Omi said, as he called on the government to speed up the construction of temporary medical facilities.

With the medical system still under strain from an influx of COVID-19 patients, much of Japan will remain under a state of emergency through Sept. 30. Government data shows slightly over half of Japan's population has been fully vaccinated so far against COVID-19.

The government seeks to ease the scope of COVID-19 restrictions under a state of emergency around November, when it aims to complete vaccinating all people who wish to be inoculated. The plan includes letting eateries provide alcohol and allowing people to travel across prefectural borders and hold big events with more attendees even if the state of emergency is still in force.

The government will experiment with the easing of restrictions in selected areas in October to see if proof of vaccination or negative COVID-19 test results required for entry can be checked smoothly, government sources said.

Tokyo and its three surrounding prefectures as well as Osaka and Fukuoka prefectures have been tapped as candidate sites, they said.

The trial, expected to involve several hundred restaurants and around 10 small concert halls, will last around a week at each venue, with local and central governments to jointly decide on what extent restrictions are eased.

The government will accept applications from interested prefectures until Friday, ultimately selecting participants from the 27 prefectures currently under a state of emergency or quasi-emergency measures. Given the interest in the project, it expects to conduct the dry run in over 10 prefectures.

But Omi has remained apprehensive about lifting restrictions too soon, telling parliament on Thursday, "We should not do such things while still under a state of emergency."