Just heard about this place. If you're a foreigner in Japan and speak very little or no Japanese, this may be your best bet without worrying about getting scammed.
https://fuzoku.sod.co.jp/sp/jhgc/en/
I'm getting the shot. Got Covid Feb 2020 before any vaccine was out. Hospitalized intensive care for 4 weeks. Thought I wasn't going to make it. Another 4 weeks out of intensive care in hospital, then home care (because they needed the bed) on oxygen for 6 months.
I don't know the science but...