I don't know about all-cause mortality, but this recent review says vitamin D supplementation definitely helps with respiratory infections, which is more german

I don't know about all-cause mortality, but this recent review says vitamin D supplementation definitely helps with respiratory infections, which is more germane at the moment. https://www.ncbi.nlm.nih.gov/pubmed/28202713?dopt=Abstract https://www.ncbi.nlm.nih.gov/pubmed/28202713?dopt=Abstract