Before the vitamin D link was so widely reported, I remember seeing a bunch of comments online (including HN) from people saying things along the lines of "We've been self-isolating at home since January, only bought things online, and STILL got sick!" I'm not a professional and this is just me spouting ideas, but I wonder if this is one reason why America is so hard hit compared to Asia. Americans generally commute…
Some examples for Thailand are included in this paper[1]. One study said this: "Soontrapa et al. [15] evaluated vitamin D status in a younger group of premenopausal women found the prevalence of vitamin D insufficiency to be 77.8%, which was as high as the rate found in elderly Thai women living in nursing homes."
There is lots of sun in Thailand, but people avoid it as much as possible, staying indoors and applying lots of sunscreen if they must go out.