Germans found that Vitamin D increases the immune system by 3-5 times and is BETTER than any vaccine at helping the immune system beat the h5n1 (bird flu) virus.
Perhaps most importantly, it dramatically stimulates the expression of potent anti-microbial peptides, which exist in neutrophils, monocytes, natural killer cells, and in epithelial cells lining the respiratory tract where they play a major role in protecting the lung from infection.
An interventional study showed that vitamin D reduces the incidence of respiratory infections in children.