There was a newspaper article (in Israel, where I live) on this subject. It noted that the two populations most mostly likely to suffer from covid are Haredim (ultra-Orthodox Jews) and Arabs, both of which tend to dress conservatively, and the latter of which have darker skin than the average Israeli, and thus might have less vitamin D in their systems.

Of course, these two populations also tend to be poorer and have larger families living together, so there are other factors to consider as well.

For example, Israelis are told to worry about skin cancer, and thus tend to use sunblock when going out in the summer. Which removes the chance of serious vitamin D generation, from what I understand.

I find the discussion of vitamin D to be very interesting, but it feels like we need a few more studies to determine whether this is the big one. But the more discussions I see, the more I think it might actually explain quite a lot.