D3 might be a proxy for age, as I thought older folks were to take D and Calcium to reduce the chance of osteoporosis. It might also be a proxy for overall health, as those short of D3 are probably not eating "well-balanced diets, ..." that marketeers are fond of talking about. It's also possible that it has a direct effect on the result, which would (as mentioned already) make it an excellent complement of vaccination. One reduces the chance of the ultimate problem, one reduces the chance of duration/problems. And it's cheap.
From the second paragraph in the abstract, it seems that they adjust for age: "Mortality rates from clinical studies were corrected for age, sex, and diabetes."
Does it adjust for other vitamins? People with sufficient vitamin D levels likely take supplements along with other vitamins so it may be that it's not just vitamin D, but it in combination with other vitamins. If this study just gave people vitamin D supplements and nothing else then it may be that it didn't have needed "supporting vitamins".
People with sufficient Vitamin D levels may also spend more time outdoors. For some, this may indicate more exercise as well. The primary source of Vitamin D naturally is sun exposure. So lifestyle factors other than eating play a large role, like time outdoors, and how far north one lives.
Had not considered this - it's a good point. There is a lot of fortified food in the N American, possibly Western European diet which includes D3 (particularly for milk drinkers). But milk drinking is limited to particular gene families as I thought it was a (quite) recent mutation...
But I would expect older people taking D3 for (pre)osteoporosis would generally be high risk for COVID. I know this thread is overflowing with anecdotal information but two family members in their sixties had completely asymptotic covid infections. Neither was generally healthy (obesity in one, asthma in the other) but both supplemented with Vitamin D3.
