So far, people who do research have come up with two very plausible answers:
- Demographics. Covid-19 is well known to affect older people more. Europe has a much higher proportion of older people.
- Vitamin D. It appears (although it’s early days yet, so don’t treat this as an established fact) that low levels of vitamin D are a risk factor for becoming severely ill. We produce vitamin D in our skin by exposure to sunlight, and “third world countries” are predominantly in areas with much more sunlight.
This would, incidentally, also explain why immigrant communities are harder hit in Europe.
Where I live, vitamin D deficiency is such a huge problem that we evolved melanin deficiency and lactase production in adulthood to overcome it. We live so far north that we get abysmal levels of sunlight—it is only even physically possible to produce vitamin D in our skin for half the year—and we had to evolve lighter skin (which makes it easier to produce vitamin D) and be able to digest milk as adults (since milk contains low levels of vitamin D; you don’t get enough for what is considered “normal” levels, but enough to survive). Children up here get free vitamin D supplements until the age of 2, and immigrants and descendants of immigrants with darker skin often need to take vitamin D supplements all year; not everyone is aware of this, though, and they often go around untreated until they seek medical advice for some other complaint.
That last bit would then be the explanation for why immigrant communities are harder hit in Europe. We all have low levels of vitamin D, but immigrants from further south often walk around with actual, but undiagnosed, deficiency.