A number of papers suggests that low vitamin D impacts a number of respiratory condition risks including mortality. As ever things are complex and more research is needed, but there is quite a large body of general evidence and results suggesting a protective factor for vitamin D in respiratory disease, and the results are thought provoking.
Consistent with a role in respiratory conditions Vitamin D appears to have a role in tight cell junction function including in the lung and digestive system.
Regulation of tight cell junction function is also a factor in spread in cancer as discussed elsewhere
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3865708/ ‘Vitamin D, vitamin D receptor and tissue barriers’
https://www.ncbi.nlm.nih.gov/pubmed/30409076 ‘Vitamin D Receptor Deletion Leads to the Destruction of Tight and Adherens Junctions in Lungs.’
(suggesting a role for Vit D in tight cell junction function).