It's not just Japan, we've seen this pattern in various countries in many phases of the pandemic, like the Delta wave in spring India and the recent Omicron wav

It's not just Japan, we've seen this pattern in various countries in many phases of the pandemic, like the Delta wave in spring India and the recent Omicron wave in South Africa which seems to have peaked. For some reason, we see waves that appear to crash and recede long before the majority of the population is no longer succeptible (either through vaccination or infection). I believe the reason(s) are still largely unknown. There are multiple plausible explanatory factors, but nothing be really validated. And especially structural explanations (local age distribution, underlying comobidities, population genetics) seem inadequate to account for the huge time variation in cases/deaths. To the point, nobody can confidently predict if Japan, India or South Africa won't have another huge wave 3 or 6 months from now.