IMPORTANT NOTE: Due to receiving a 2-year partial funding enabling only a single-centre pilot trial, rather than an adequately-powered multicentre study, the primary outcome was modified post hoc for the overall change from baseline in total serum 25OHD during the study as well as at 3.5 and 7 months, similar to our previous pilot study.(NCT01999907) Post hoc secondary outcomes included the group difference in the proportion of children with total 25OHD ≥75 nmol/L at 3.5 and 7 months and in the rate of oral corticosteroid courses per child. Other a priory specified outcomes included the proportion of children with hypercalciuria (Ca:Cr) \>1.25 (1-2 years), \>1 (2-5 years) nmol/nmol at any point in time; proportion of children with ≥1 exacerbation requiring rescue oral steroids (former primary outcome); number of emergency department (ED) visits; intensity and duration of asthma symptoms and cumulative use of rescue ß2-agonist use, documented on Asthma Flare-up Diary for Young Children (ADYC); the impact of parents' functional status during exacerbations ascertained on the Effect of a child's asthma flare-up on parents; and duration of URTI.

Based on this analysis of this new post-hoc primary outcome, we have changed the intervention and the primary outcome and obtained funding for a new large multicentre study NCT03365687. it is thus important to share the results of this current trial with other investigators

PRIOR REPORTED DESCRIPTION Design: A multicenter triple-blind randomized parallel-group, placebo-controlled trial of vitamin D3 supplementation. Children aged 1-5 years with (i) physician-diagnosed asthma, predominantly triggered by upper respiratory tract infections (URTIs), (ii) ≥4 reported URTIs in the past year, and (iii) ≥1 exacerbation requiring OCS (a recognised marker of moderate and severe exacerbations) in the past 6 months or ≥2 in the past 12 months, will be randomly allocated to one of two treatments in blocks of 4-6, stratified on recruitment site: Intervention group (n=432)-100,000 IU oral vitamin D3; control group (n=432)-identical placebo, for 2 oral doses, 3.5 months apart. Co-intervention with asthma therapy (preventive or pre-emptive ICS) will be left to the discretion of the physician and documented. Children will be followed every 3.5 months as per usual practice, with a home visit 10 days after each bolus, during which urine and blood will be sampled for urinary calcium:creatinine ratio, serum vitamin D, markers of calcium metabolism, and mechanistic exploration. A validated diary will serve to document the intensity and severity of exacerbations. In two sites, preschool lung function will be documented (if ≥ 3yrs). Outcomes: Primary endpoint-number of exacerbations requiring rescue oral corticosteroids (OCS) per child, documented by medical and pharmacy records. Secondary outcomes: duration and severity of exacerbations (symptoms \& β2-agonist use, by diary; emergency visits, by medical records), parental functional status (by validated instrument), asthma therapy intensification and health care and direct costs (by health records \& parent reports). Safety, mechanistic, exploratory outcomes: hypercalciuria, calcium metabolism, excess vitamin D, change in serum gene expression at 10 days (first 25 patients); and change from baseline at 7 months in preschool lung function (2 sites). Based on 3 published RCTs, a sample of 432 per arm (400+7.5% attrition) will provide 80% power (2-tailed alpha of 5%) to detect a 25% reduction in number of exacerbations requiring OCS/child (0.55 vs. 0.4125).