A double-blind, randomized, clinical trial of the effect of vitamin A and zinc supplementation on diarrhea and respiratory tract infections in children in Mexico City, Mexico.  Long et al., 2006 [275]  Mexico  736 children living in a peri-urban area  Vitamin A vs. 20 mg zinc daily vs. vitamin A + zinc vs. placebo for 12 months  No effect on the incidence of respiratory infections.