The USA and Vietnam have almost the same life-expectancy stats. I don't think one can say the difference in health is "an order of magnitude".

Obesity in Vietnam is 1%...lowest in the world. Versus US with ~42%.

We know the seriousness of obesity wrt covid.

Seems like this relationship would be very easy to discover in the worldwide statistics.

If this was the decisive factor, you'd expect Belgium, Italy and France to have low rates, with Germany, the UK, and most of Eastern Europe having high rates.

This turns out not to be the case.

Maybe not - but I'd guess that some combination of lack of chronic disease like obesity/diabetes on top of mask usage helped a lot.

If the lack of chronic disease leads to less severe covid, you'd expect the nosocomial spread factor of the pandemic is lower.