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.
You're asking me to do a significant amount of work here, and it's not clear you understand the actual claims being made. "Vitamin D weakens defenses against SARS-CoV-2" is not a good summary of them.
Thanks for clearing that up for me, I've read LessWrong for years but haven't tried to post much before the virus gave me some free time, I'm still getting used to the format.
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.
As far as lung health goes, tuberculosis prevalence is inversely correlated with B3 (of any kind, not just NR). This isn't some $@#! malaria drug or Ebola has-been, there's no side-effect downside.
This is a link to a post on my blog regarding seasonality of COVID-19, and its implications for some low cost but possibly high impact interventions.