Vitamin D deficiency is a strong factor, not for getting Covid-19, but for bad outcomes.
It's especially common for people with dark skin, since that blocks the ability of producing Vitamin D from sunlight.
If that's part of what's happening to the Navajo, I don't know, but you should probably eat Vitamin D pills, especially during this indoor lockdown, and if if you have dark skin.
It might be a strong factor - this isn't yet demonstrated conclusively by enough peer reviewed studies yet, so it shouldnt be described as conclusive. Thankfully, there a long history of vitamin D supplementation, so unlike hydroxychloroquine, it isn't likely to increase death rates. At worst it might just do nothing.
Also, a number of common foods contain or are supplemented with vitamin D. Assuming that somebody has no fundamental issues with vitamin D absorption from food, a typical healthy diet that includes some of those foods should be sufficient to keep a person out of vitamin D deficiency, with no pill-based supplement needed.
A "typical healthy diet" can provide even less vit D since much of the vit D consumed it from fortified milk and breakfast cereals.
I think a lot of people would benefit from boosters right now. As you say, it does no harm.
https://youtu.be/4_ZJ8YDOX6g?t=1771
Vitamin D insufficiency has been strongly correlated to COVD-19 deaths and severe symptoms in various locations, although obviously more data is still needed to prove causation.
Either way unless you work outside you should probably be on 5000 IU of Vitamin D a day just to compensate for sunlight deprivation.
"There is no strong scientific evidence to show that very high intakes (ie, mega supplements) of vitamin D will be beneficial in preventing or treating COVID-19."
Strong scientific evidence usually takes a lot longer than that to establish.
Also: I'm not advocating "very high" doses. A normal daily supplement dose should be just fine.
It's not good for people to be just advising "eat vitamin D" though.
People should consult a doctor to determine a) to what extent they are deficient (or not), and b) that they have no health issues that make taking vitamin D dangerous.
"Get outside more", is temporarily bad advice: "eat vit-d instead" is temporarily good advice.
Your doctor has more important things to do.
The Navajo have been in the general area (the US forcibly moved them, but the latitudes are similar) of where they are now for a long time. Their skin tone should be well-adapted to the region, unless they're not going outside.
If they're living a regular American lifestyle, which often produces Vitamin D deficiency even for the palest white people, they will be affected even more.
But I mostly intended this as general advice for the HN readership.