Since I wrote up the simple analysis I did comparing Covid-19 prevalence across countries, there have been a few official studies published. They have shown similar conclusions to mine, but done with more precise data and more rigor. The health of the population seems to have made a big difference to mortality rates, with obesity rates being one factor that I had looked at. One study used vitamin D levels as a predictor of a population’s ability to fight off the effects of the disease. (See chart reproduced below.) What surprised me about this study was not the effect, but the countries whose populations had high vitamin D levels and those that had low levels. Most of us know that vitamin D is in part produced by our skin when in sunshine, so I presumed that southern European countries would have decent vitamin D levels and northern Europe maybe less good. In fact, it’s the other way around. Darker skin produces less vitamin D than pale skin, and if the weather is so hot you don’t go outside in the middle of the day, you can’t produce much vitamin D. Plus, you can boost your vitamin D levels by eating, among other things, oily fish. So Scandinavia came out top of the league for vitamin D levels and they’ve had far fewer deaths than most other countries; even Sweden with it’s no-lockdown policy.
A research paper which goes into detail on this study is available here.
The chart below is taken from the Irish Medical Journal, volume 113, no 5, page 81, showing higher mortality rates associated with lower vitamin D levels.