I think that, like in Florida, a big component in the recent collapse in case numbers in Japan (especially Tokyo), is that the Delta variant has had its run, and burned out naturally. It goes quicker in densely populated areas, and Tokyo is famously densely populated.
The vaccines seem more effective in mitigating serious illness and reducing death from Delta, than they are at preventing infection or transmission.
Even though the vaccines may be even less of a fit for the highly mutated Omicron variant, they are still likely of value in mitigating adverse outcomes.
I just went and got a booster myself, in preparation for Omicron having its run (and Delta still on the move in my area). To my knowledge, I have not yet had a COVID-19 infection, but I figure it is only a matter of time.
Others have speculated that the dramatic collapse in Tokyo case numbers may be because of widespread use of Ivermectin, but my understanding is that it is not widely used there for prevention, and I have no idea how common it is for use in treatment. Have you heard anything about Ivermectin use (actual rates) in Japan?
Ivermectin is not listed by the Japanese government as an approved medicine to treat the corona virus,