Seasonality of COVID-19, Other Coronaviruses, and Influenza

Look up how much vitamin D you get from an hour of full-body sun exposure... if Vitamin D weakened defense against SARS-CoV-2 (instead of helping, as it does against other viruses), the Australians and Indians would have trouble ;)

...when actually they're doing much better than the US.

I would like to call attention to the conflict of interest statement at the end, where the senior author Charles Brenner is identified as chief scientific adviser of ChromaDex, maker of Tru Niagen, a profitable nicotinamide riboside supplement. I'm not saying the theory is necessarily wrong--NAD+ is implicated in many aspects of aging, and aging is obviously a risk factor for Covid mortality. But the effects of NR supplementation in humans have been a bit over-hyped in the past. Again, I don't mean to imply that it does nothing, but it has been pushed as an anti-aging supplement without good evidence for improvement of biomarkers other than NAD+. For instance this study found no effect on insulin sensitivity or other metabolic measures.