The evidence points in a positive direction: vitamin D influences the immune system through well-described biological mechanisms, and supplementation reduces the risk of respiratory infections and possibly a severe course of COVID-19, but exclusively or primarily in people with a vitamin D deficiency. The effect is modest and the clinical studies are sufficiently consistent to say that it is not overrated, yet neither is it spectacular. Those whose levels are already in order have little to gain from additional supplementation.
Vitamin D has a direct biological pathway to the immune system. Immune cells such as T cells, B cells, monocytes and antigen-presenting cells all possess vitamin D receptors as well as the enzymes needed to activate vitamin D themselves. This has been well established in laboratory research. Once activated, vitamin D stimulates immune cells such as macrophages to produce antimicrobial peptides -- substances that directly attack bacteria and viruses -- and promotes autophagy: the cellular clean-up process by which cells neutralise pathogens such as the tuberculosis bacterium. The biological foundation for an immune role is therefore solid.
At the same time, the broader picture is more nuanced. Observational studies and meta-analyses show a clear association between low vitamin D levels and a higher risk of acute respiratory infections. Meta-analyses of randomised trials confirm that supplementation can have a protective effect, but this effect is modest. It is most pronounced in people with a demonstrable deficiency and in children. Those who already have an adequate vitamin D level have little reason, based on the available studies, to expect benefit from particularly high doses.
In the context of COVID-19, clinical studies show that correcting a vitamin D deficiency reduces the risk of hospitalisation, admission to the intensive care unit and death. The researchers explicitly emphasise that this concerns correcting a deficiency, not taking megadoses on top of an already adequate level.
For autoimmune diseases, including type 1 diabetes, multiple sclerosis, inflammatory bowel disease and lupus, meta-analyses of observational studies show an association with low vitamin D levels. Laboratory and animal studies demonstrate how vitamin D can dampen excessive immune responses and promote regulatory mechanisms. Clinical trials in type 1 diabetes and inflammatory bowel disease show modest benefits from supplementation, but a fully proven causal relationship has not yet been established.
Also relevant is how widespread deficiency actually is. Research from the American NHANES database shows that 61% of white and 91% of Black Americans had an insufficient level (below 32 ng/mL). Deficiency is therefore far from rare. In practical terms, if someone has symptoms or is at elevated risk, it is sensible to have their vitamin D level measured and, if a deficiency is confirmed, to supplement, rather than taking high doses on a whim.
Based on multiple meta-analyses and systematic reviews (PMIDs 36857810, 35507293, 34322844, 23857223), supplemented by a large prevalence study (PMID 18460265). The immune mechanisms are well described in laboratory studies; the clinical effects on infections are moderate to modest in magnitude but consistent across multiple RCT meta-analyses.