Conclusions Vitamin D supplementation  was safe and it protected against acute respiratory tract infection overall .
Patients who were very vitamin D deficient and those not receiving bolus doses experienced the most benefit .
Objectives To assess the overall effect of vitamin D supplementation on  risk of acute respiratory tract infection  , and to identify factors modifying this effect .
Randomized Trial of Vitamin D Supplementation and Risk of Acute Respiratory Infection in Mongolia
OBJECTIVE : Observational studies suggest that serum levels of 25-hydroxyvitamin D ( 25[OH]D ) are inversely associated with acute respiratory infections ( ARIs ) . We hypothesized that vitamin D supplementation of children with vitamin D deficiency would lower the risk of ARIs . METHODS : By using cluster r and omization , classrooms of 744 Mongolian schoolchildren were r and omly assigned to different treatments in winter ( January – March ) . This analysis focused on a subset of 247 children who were assigned to daily ingestion of unfortified regular milk ( control ; n = 104 ) or milk fortified with 300 IU of vitamin D3 ( n = 143 ) . This comparison was double-blinded . The primary outcome was the number of parent-reported ARIs over the past 3 months . RESULTS : At baseline , the median serum 25(OH)D level was 7 ng/mL ( interquartile range : 5–10 ng/mL ) . At the end of the trial , follow-up was 99 % ( n = 244 ) , and the median 25(OH)D levels of children in the control versus vitamin D groups was significantly different ( 7 vs 19 ng/mL ; P < .001 ) . Compared with controls , children receiving vitamin D reported significantly fewer ARIs during the study period ( mean : 0.80 vs 0.45 ; P = .047 ) , with a rate ratio of 0.52 ( 95 % confidence interval : 0.31–0.89 ) . Adjusting for age , gender , and history of wheezing , vitamin D continued to halve the risk of ARI ( rate ratio : 0.50 [ 95 % confidence interval : 0.28–0.88 ] ) . Similar results were found among children either below or above the median 25(OH)D level at baseline ( rate ratio : 0.41 vs 0.57 ; Pinteraction = .27 ) . CONCLUSIONS : Vitamin D supplementation significantly reduced the risk of ARIs in winter among Mongolian children with vitamin D deficiency
Conclusions Vitamin D supplementation  was safe and it protected against acute respiratory tract infection overall .
Patients who were very vitamin D deficient and those not receiving bolus doses experienced the most benefit .
Objectives To assess the overall effect of vitamin D supplementation on  risk of acute respiratory tract infection  , and to identify factors modifying this effect .