Probiotics and vitamin C for the prevention of respiratory tract infections in children attending preschool: a randomised controlled pilot study.  Ďuračková Z 2015 [69]  ChildrenRCT  Vitamin C (50 mg/day for 6 months)  Reduced incidence (p = 0.002) and duration (p = 0.006) of URTIs. No significant differences in the duration or incidence of LRTIs.