I don't think it can happen. On one of the Japanese travel podcasts I listen to, the speakers seemed skeptical that it would even open by December. I agree with those prior posters ... the vaccination rate is low, they even cut back on how many people could travel with athletes to the Olympics (assuming they happen at all), towns that were supposed to host athletes prior to the games have backed out.
So that's why I'm trying to decide on someplace else to use the air vouchers from my cancelled 2020 Japan trip. Would love to just go to Japan, but I have zero confidence it'll be open this summer, and only the barest hope it'll open by December.
ETA: I was going to go independently, not with
ABD, but that seems irrelevant to whether Japan will let you in. Country of origin could be relevant, maybe vaccination status, but I've heard nothing optimistic.
ETA (2nd): Coincidentally, I was watching NHK World Japan just now, and they just said "Officials aim to inoculate the country's 36 million seniors by the end of July." (Quoted from NHK's website b/c I was not really paying attention to the TV and wanted to make sure it was what I thought I'd heard.)