No vaccines, that I am aware of, currently authorized anywhere in the world prevent infection or transmission of the currently circulating omicron strains, for most people. At best they reduce hospitalization and seriousness.

China is having the very large waves now because they locked down for so long. Thus they had a very small population with pre-existing exposure to the omicron strains. So when some BA.5 derived omicron strains starting getting into the Chinese population a couple of months ago they started taking off in the very large, very crowded (by international standards) cities.

Eventually enough people will be exposed to these new strains that their immune systems will be able to fight back and prevent reinfection and then we should see the shallower waves that are occurring in other countries now.

Compare what's happening now to China with their first Omicron wave to what happened to the US with their first Omicron wave: https://covid.cdc.gov/covid-data-tracker/#trends_weeklycases...

Appreciate the insight

Sure. It's possible that updated vaccines could have reduced the wave. I just don't know enough to guess.

I've read that XBB.1.5 is about as different from BA.5 as BA.5 is from the original Wuhan strain. So even with this wave of infection China could get another wave of an XBB variant within a few months.

So why, then, is Japan having cases/deaths at their all time highs?

Because Japan (and Australia, and New Zealand) were also good at keeping a lid on infection spread, just like China? And now, with the new Omicron variants, they aren't?