Vitamin D has been investigated for its potential role in supporting immune function, including its effects on respiratory tract infections such as influenza. Several observational studies have found that individuals with lower levels of vitamin D are at increased risk of developing acute respiratory infections. There are also randomized controlled trials (RCTs) and meta-analyses that suggest vitamin D supplementation can reduce the risk of respiratory tract infections, particularly in people who are deficient in vitamin D. For example, a 2017 meta-analysis published in BMJ examined 25 RCTs and found that vitamin D supplementation was safe and protected against acute respiratory tract infection overall, with the greatest benefit seen in those who were very deficient.

However, the evidence specifically for influenza is more limited. Some studies, such as a 2010 RCT in Japanese schoolchildren, showed that daily vitamin D supplementation reduced the incidence of influenza A. Other trials have shown mixed or inconclusive results. While there is scientific rationale—given vitamin D's role in modulating the immune system—most guidelines do not currently recommend vitamin D solely for the prevention or treatment of influenza, except in cases of known deficiency. Therefore, the use of vitamin D for influenza is supported by moderate-quality scientific evidence, particularly in deficient individuals, but is not universally endorsed as a treatment or preventative measure for influenza alone.