Japan's state-run mass coronavirus vaccination centers plan to start inoculating people aged under 65 as early as Thursday in an expanded drive to fill in vacant slots, a government source said Tuesday.

The centers in Tokyo and Osaka were set up last month by the Defense Ministry to give shots to people aged 65 and older living in seven prefectures in the metropolitan and Kansai areas.

But as slots remained largely vacant for two weeks through June 27, the ministry last week expanded the scope to accept people from nationwide. To not waste COVID-19 vaccines, it is now considering also removing the age restrictions to allow people aged between 18 and 64 to receive shots at the centers.

The ministry is expected to make a formal decision later in the day, the source said.

The centers, which can inoculate up to 10,000 people a day in Tokyo and 5,000 at the Osaka venue, will administer U.S. pharmaceutical company Moderna Inc's two-dose vaccine, which is recommended for those aged 18 and older.

At the Tokyo center, about 87,000 of the 120,000 vaccination slots for the two-week period from Monday were unfilled as of 10 a.m. Tuesday, while about 33,000 of the 60,000 slots remained open at the Osaka center, according to the ministry.

The slots from July 28 onward are mostly filled by elderly people for their second-round shots.

Inoculations at the vaccination centers, run by the Self-Defense Forces, come in tandem with shots being administered at municipalities nationwide, mainly to health care workers and the elderly.

Some municipalities have already started vaccinations of people aged under 65 at their own venues.

Companies and universities were recently given the all-clear to administer vaccinations on site, with major airliners leading the initiative.