Thanks for taking the time to ask. Yes, scientists do think lack of sunlight during winter times is one of the factors contributing to rapid virus spread, which I’ve written about here.

Second, I also pondered if vitamin D could be a sign of severe COVID-19, not a cause. Maybe this is true, but there’s also evidence suggesting that vitamin D supplementation help protect against respiratory tract infections. And vitamin D deficiency is known biochemical to impair the immune system. But we’ll know for sure when the clinical trial is completed :)