"Put simply, medics found that severely ill flu patients nursed outdoors recovered better than those treated indoors. A combination of fresh air and sunlight seems to have prevented deaths among patients; and infections among medical staff.[1] There is scientific support for this. Research shows that outdoor air is a natural disinfectant. Fresh air can kill the flu virus and other harmful germs. Equally, sunlight is germicidal and there is now evidence it can kill the flu virus."
[1]: https://medium.com/@ra.hobday/coronavirus-and-the-sun-a-less...
I read that vitamin D and observed benefits for those who have vitamin D is a correlation. Meaning that taking vitamin D supplements might not be as helpful as getting sunlight (a natural way to get vitamin D)
With regards to sunlight exposure, sunbathing in the alps was a go-to tuberculosis treatment prior to having antibiotics.
Vitamin D may have modulatory effects on the biochemical pathways linked in the original article. Most pharmaceutical products we study aren't going to directly kill bacteria or viruses, usually they have a particular physical effect on a specific molecule that then leads, through various cellular pathways, to the death or suppression of the pathogen. I highly doubt Vitamin D directly kills anything, it could modulate symptoms or the pathophysiology of COVID-19 the disease, vs. SARS-COV-2 the virus itself.
If "it" means UV light, yes there are UV sterilizers aplenty today. If "it" means outdoors/fresh air, there are a lot of components to that and it's not like "fresh air" enters "in vivo" per se.
Yes, I'm aware of UV sterilizer, but does it work when the microorganisms are inside you? You don't want to get an "internal tan"
Very very interesting theories as it also helped during the 1918 flu.