RCT: Vitamin D supplementation for older adults (mean age 67 years) for 5 years reduced the risk of autoimmune diseases by 22%. Absolute risk in the treatment group was 0.95% vs. 1.19% in the placebo group.

Vitamin D and marine omega 3 fatty acid supplementation and incident autoimmune disease: VITAL randomized controlled trial – The BMJ

Commentary: Vitamin D and fish oil supplements may help prevent autoimmune disease, study says – CNN

Related:

RCT: Oral Vitamin D supplementation (60.000 IU per month) did not reduce mortality in patients 60 years or older (unscreened individuals in Australia, many without insufficiency, average 30ng/mL). Exploratory analyses pointed towards an increased risk of death from cancer.

RCT: Vitamin D supplementation is not beneficial for the prevention of cardiovascular disease and cancer.

USPSTF Draft Statement: Insufficient evidence to recommend multivitamin supplements for the prevention of CVD or cancer. The statement also recommends against the use of beta-carotene (increases risk of CVD mortality and lung cancer) or vitamin E (clear evidence of no benefit)

USPSTF Statement: Insufficient evidence to recommend Vitamin D deficiency screening

Vitamin D, calcium, other vitamins, and supplements do not prevent cardiovascular diseases