INTERPRETATION Results indicate that vitamin D has a protective effect against RTI , and dosing once-daily seems most effective .
BACKGROUND Low levels of 25-OH vitamin D are associated with  respiratory tract infection ( RTI ) .
However , results from r and omized controlled trials are inconclusive .
Therefore , we performed a systematic review and meta- analysis to assess the preventive effect of vitamin D supplementation on RTI .
Effect on the incidence of pneumonia of vitamin D supplementation by quarterly bolus dose to infants in Kabul: a randomised controlled superiority trial
Summary Background Vitamin D has a role in regulating immune function , and its deficiency is a suggested risk factor for childhood pneumonia . Our aim was to assess whether oral supplementation of vitamin D3 ( cholecalciferol ) will reduce the incidence and severity of pneumonia in a high-risk infant population . Methods We did a r and omised placebo-controlled trial to compare oral 100 000 IU ( 2·5 mg ) vitamin D3 with placebo given to children aged 1–11 months in Kabul , Afghanistan . R and omisation was by use of a computer-generated list . Vitamin D or placebo was given by fieldworkers once every 3 months for 18 months . Children presenting at the study hospital with signs of pneumonia had their diagnosis confirmed radiographically . Our primary outcome was the first or only episode of radiologically confirmed pneumonia . Our analysis was by intention to treat . This study is registered with Clinical Trials.gov , number NCT00548379 . Findings 1524 children were assigned to receive vitamin D3 and 1522 placebo .  There was no significant difference between the incidence of first or only pneumonia between the vitamin D ( 0·145 per child per year , 95 % CI 0·129–0·164 ) and the placebo group ( 0.137 , 0·121–0·155 ) ; the incidence rate ratio was 1·06 ( 95 % CI 0·89–1·27 ) . From 652 children during five separate periods of testing serum calcifediol , only one child in each of two testing periods had results greater than 375 nmol/L in the intervention group — a toxic level . Interpretations Quarterly bolus doses of oral vitamin D3 supplementation to infants are not an effective intervention to reduce the incidence of pneumonia in infants in this setting . Funding Wellcome Trust and British Council
INTERPRETATION Results indicate that vitamin D has a protective effect against RTI , and dosing once-daily seems most effective .
BACKGROUND Low levels of 25-OH vitamin D are associated with  respiratory tract infection ( RTI ) .
However , results from r and omized controlled trials are inconclusive .
Therefore , we performed a systematic review and meta- analysis to assess the preventive effect of vitamin D supplementation on RTI .