And note, two caveats: 1) it’s a preprint study, so it hasn’t undergone peer review yet 2) it doesn’t control for age
A better study would control for age, as the elderly often have low vitamin D due to being indoors a lot and having aging skin that doesn’t make vitamin D as well. However, these results are consistent with current knowledge about vitamin D and immunity. I imagine that even if you controlled for age, you would still see a significant effect.
