Interesting and new article, written by the Syracuse University Department of Chemistry and published 6 days ago.
papers.ssrn.com/sol3/Papers.cfm?abstract_id=3561958
Key data is a graph of latitude versus deaths per 1000 cases reported.
Conclusion: ''COVID-19 morbidity and mortality are correlated with vitamin D deficiency......... Indeed, the raw data from the World Health Organization show that barely 1% of the reported deaths worldwide occurred in individuals south of the Tropic of Cancer''
Peak death rate is very clearly between the 40th and 50th parallel north latitudes. Note that the following regions lie in this geographical region: London, New York, Lombardy and Madrid.
Notable decrease in Norway, Sweden and Denmark latitudes: ''Scandinavian nations have some of the lowest vitamin-D deficiency rates on Earth ............attributed mainly to dietary supplementation, with both fish liver oils and vitamin-D-fortified milk.''
Indonesia and Philippines seem to show higher than expected death rates perhaps due to higher rates of vitamin D deficiency and limited testing in high population densities.
Overall year-round sunny nations seem generally to have lower death rates, perhaps due to a combination of sunlight exposure and vitamin D levels and also the viral instability above 28 degrees C combined with ultra violet rays.