Huge covid news here if this paper is correct. Japan has been a mystery for some time now looks like they had large spread and just no excess deaths.

https://www.medrxiv.org/content/10.1101/2020.09.21.20198796v1

This isn't necessarily unexpected. It was speculated in a few places that a native mild corona-virus circulating in a population could give cross resistance. Not enough to make you immune but enough to make the disease mild. This now looks to be a good explanation for Japan.

The covid sceptics will doubtless claim that this shows the IFR is low, that is a wrong conclusion, because it is using the exact same sero-tests that show that the observed IFR in UK and Spain was > 1.5%. Accepting this result accepts that even mild cases show up in serosurveys.

The optimistic take is that this means that a live virus cow-pox/small pox style vaccine is possible if researchers can isolate exactly which circulating virus gives cross resistance, although this is still speculation for now. Would like to see more Japan surveys.

Of course, the paper could just be a biased or unlikely sample of people, so I do not think we can definitively conclude that the sero-positivity in Toyko is as high as this paper claims. Because it was from locations it is not a true random sample.