The correlation is making more and more sense. Vitamin D plays a huge role in immune system. There is more than one role. The least understood and latest link is to a macrophage stimulating factor.
The key to complete immune response is tagging of the antigen or invasive protein, peptide, particle.
Without tagging an activated killer cell cannot migrate to the spot it needs to. This is why so many tumor vaccines fail. There is no tagging.
As we spend less time outdoors, as our diets trended toward carbohydrates and grains, inflammation increased, parasitic and yeast infections became more virulent due to higher blood sugars, and vitamin d levels plummet.
Many of these organisms actively break down vitamin d and macrophage stimulating factor. Cancer does too.