Summary
A review composed by several American and Canadian research institutes gave an overview of studies on vitamin D evaluating its influence on immunity and its function as an immunomodulator. Vitamin D deficiency often occurs in the winter season as a result of decreased sunlight and sun exposure. Several studies showed a clear relation between vitamin D deficiency and, for instance, respiratory infections. Some studies showed that increased levels of vitamin D reduced the incidence of (viral) respiratory tract infections in children. More details can be found in the review article.
References
Cannell et al.