This review, conducted by Swiss researchers, gives an overview of several studies investigating the immune-enhancing effect of vitamin C and zinc and their influence on clinical conditions. The results showed that vitamin C concentrations rapidly decline during infections and that supplementation of vitamin C improves the function of the human immune system. Zinc deficiency also showed an impaired immune function. According to several studies, both micronutrients have essential roles in immune function and induce a reduced risk, incidence, severity, and duration of infectious diseases, like respiratory diseases. Further details can be found in the study.
Wintergerst et al.