In March, The American Journal of Clinical Nutrition published the results of a double-blind, placebo-controlled study performed on school-aged children that found that children who supplemented their diets with vitamin D during the winter months demonstrated significantly lower incidences of influenza than children who did not supplement with vitamin D.
It also seemed to reduce asthma attacks.
Sounds worth the try!
Source: Randomized trial of vitamin D supplementation to prevent seasonal influenza A in schoolchildren