We know that obesity is a large driver not just in severity, but transmission.
When you compare a world map of obesity rates to Covid death rates, you'll find a better match than any other single variable, far better-matching than income, masking, density, or any other single variable.
Just a stab in the dark, but generally high social conformity and particularly mask wearing.
There's a trial study currently in progress in Japan to investigate this, but all the evidence we have points to ivermectin being of no benefit to Covid patients unless they have parasitic worms. See, for example: https://astralcodexten.substack.com/p/ivermectin-much-more-t...
It's not an unanswered question, in other words. Ivermectin doesn't help against Covid.
Wearing face masks in public places.. indoors and outdoors. Social distancing is non existent considering how crowded places can be.
The same story applies to China, Hong Kong, Macau and S. Korea as well as some countries in SE Asia like Vietnam. Face mask wearing is culturally acceptable/normal in this part of the world so that's why people there started wearing them as soon as stuff started kicking off in Jan 2020 without their governments telling them.
People on HN are arguing over scientific papers on whether face masks are effective. You don't need scientific papers when these countries are proof that face mask wearing is effective.
Less people infected, lower chance of mutations. You will not hear about new SARS-COV-2 variants first being detected in these countries.
RNA viruses have high mutation rates and variants will never end in the same way that flu & common-cold variation continues. The more interesting question is how will our immune systems adapt and will vaccines continue to provide protection against severity of illness at the same time as they apply selective pressure that enhances viral transmissibility?
Of further interest is why Japanese and Taiwan infection and severity rates remain comparatively low and what - if any - role their openness to a certain anti-parasitic, protease inhibiting drug has played in their success.
Also of great interest is why aspiration isn't mandatory in the US to prevent the vaccine from entering the bloodstream (primarily the heart) rapidly and in large quantities.
Many unanswered questions.