A clinical trial led by Mitsuyoshi Urashima and conducted by the Division of Molecular Epidemiology in the the Department of Pediatrics at the Jikei University School of Medicine Minato-ku in Tokyo found that vitamin D was extremely effective at halting influenza infections in children. The trial appears in the March, 2010 issue of the American Journal of Clinical Nutrition (Am J Clin Nutr (March 10, 2010). doi:10.3945/ajcn.2009.29094)
This means vitamin D appears to be 800% more effective than vaccines at preventing influenza infections in children.