I agree that it will take a while to get non "hot takes" but in the meantime, agree that speculation can be fun. I'd previously spent a lot of time in Japan visiting, but this past year has been my first really "living" here so it's been a bit eye-opening to see first-hand some of the extreme dichotomies of bureaucratic competence/incompetence during this period. I have a few theories of my own on Japan's low numbers:

* There's probably consistently an undercount (in my head, I just multiply the daily numbers by 5-10X), but not much more than that as we saw in Dec-Jan that a true spike will be reflected by hospital overflows (https://www.stopcovid19.jp/ has been my go-to for tracking hospitalization numbers). There's some finagling of the death toll as well (COVID-19 deaths reported as pneumonia) but I think we'll get a true accounting eventually with excess mortality modeling (offset by much lower flu) that won't be so far off to matter. [1][2] Overall, somehow, the numbers really have been pretty low. This is corroborated by most seroprevalence research. [3][4]

* There seems to have been a lot more spread amongst young people in Tokyo/Osaka, especially over the summer but it seems to be fairly well contained - I think that metabolic health does play a big role there in why there wasn't a big spike in serious cases (obesity, and maybe more importantly, hypovitaminosis D is significantly lower in Japan). As to why it didn't spread beyond that group like most other places, I believe that the lower rate of shared housing is likely a big factor - single person households makes up an amazing (and still increasing) 35% of the population. Anecdotally, there seems to have been a conscious decision by many people to not see older family members to put them at risk, and there's been some interesting (but not conclusive) research on Japanese lockdown behaviors that seem to differ a bit from what I've seen in the West. [5][6]

* While there are exceptions, again, totally anecdotally, I've seen a huge decrease in people publicly coughing, sneezing etc and when there have been people coughing in public, generally people were well, avoiding them like they had the plague. In any case, I think an early focus on ventilation and near universal masking was pretty successful in significantly lowering casual spread.

* One thing that hasn't been reported much, but Bromhexine and Ambroxol are both very common OTC cold medicines in Japan. These are TMPRSS2 inhibitors, blocking the primary entry path for the SARS-CoV-2 virus (the secondary path, endosomal entry, can be blocked by Nitazoxanide or Niclosamide). It's quite possible that in Japan, people have been unknowingly taking medicine that has been effective in moderating COVID-19 severity (this to me, seems more likely than any proposed variolation effect).[7] To me, one of the frustrating things over the past few months is that there's been enough research pointing towards many low-risk prophylactics/treatments that are probably effective for preventing severe COVID that have been pretty largely ignored.

[1] Yorifuji, Takashi, Naomi Matsumoto, and Soshi Takao. “Excess All-Cause Mortality During the COVID-19 Outbreak in Japan.” Journal of Epidemiology 31, no. 1 (January 5, 2021): 90–92. https://doi.org/10.2188/jea.JE20200492.

[2] Kurita, Junko, Tamie Sugawara, Yoshiyuki Sugishita, and Yasushi Ohkusa. “Excess Mortality Probably Due to COVID-19 in Tokyo, Japan in August and September, 2020.” MedRxiv, February 9, 2021, 2020.07.09.20143164. https://doi.org/10.1101/2020.07.09.20143164.

[3] Yoshiyama, Takashi, Yasuki Saito, Kunitsugu Masuda, Yoshiko Nakanishi, Yasutoshi Kido, Kazuhiro Uchimura, Satoshi Mitarai, et al. “Prevalence of SARS-CoV-2–Specific Antibodies, Japan, June 2020 - Volume 27, Number 2—February 2021 - Emerging Infectious Diseases Journal - CDC.” Accessed March 11, 2021. https://doi.org/10.3201/eid2702.204088.

[4] Tanaka, Akihito, Shohei Yamamoto, Kengo Miyo, Tetsuya Mizoue, Kenji Maeda, Wataru Sugiura, Hiroaki Mitsuya, Haruhito Sugiyama, and Norio Ohmagari. “Seroprevalence of Antibodies against SARS-CoV-2 in a Large National Hospital and Affiliated Facility in Tokyo, Japan.” The Journal of Infection. Accessed March 11, 2021. https://doi.org/10.1016/j.jinf.2021.01.010.

[5] CARF:Center for Advanced Research in Finance. “Japan’s Voluntary Lockdown.” https://www.carf.e.u-tokyo.ac.jp/en/research/5603/.

[6] Muto, Kaori, Isamu Yamamoto, Miwako Nagasu, Mikihito Tanaka, and Koji Wada. “Japanese Citizens’ Behavioral Changes and Preparedness against COVID-19: An Online Survey during the Early Phase of the Pandemic.” PLOS ONE 15, no. 6 (June 11, 2020): e0234292. https://doi.org/10.1371/journal.pone.0234292.

[7] Gandhi, Monica, and George W. Rutherford. “Facial Masking for Covid-19 — Potential for ‘Variolation’ as We Await a Vaccine.” New England Journal of Medicine 383, no. 18 (October 29, 2020): e101. https://doi.org/10.1056/NEJMp2026913.