Vitamin D In the Prevention of Viral-induced Asthma in Preschoolers

This is a multicenter triple-blind randomized parallel-group, placebo-controlled trial of vitamin D3 supplementation. Children aged 1-5 (\<6) years with physician-diagnosed asthma predominantly triggered by upper respiratory tract infections will be screened for enrolment in paediatric asthma, respiratory or allergy clinics and the ED departments and randomized between Sept 1 to January 31, annually (4 recruitment years) and year around from 2022 onwards. Using a computer-generated random list, stratified by site, children will be allocated (1:1) using permuted block randomisation method to enhance concealment. Children will be followed for 7 months, with 3 visits every 3.5 months with repeated urine (for calcium:creatinine ratio) and blood samples. In addition, ten (10) days after each bolus, urine will be sampled for urinary calcium:creatinine ratio. In case of elevated urine calcium:creatinine ratio, a blood sample may be needed primarily for markers of calcium metabolism and exploratory outcomes. Only patients enrolled at CHU Sainte-Justine and Montreal Children's Hospital will receive a systematic home visit 10 days after first bolus for both urine and blood samples. There will be 6 follow-up phone calls, at week 1 and then monthly, to inquire about exacerbations and URTIs, remind parents to complete questionnaires and to collect a nasal swab at each exacerbation and screen for adverse events. The main outcome is the number of courses of rescue oral corticosteroids (OCS) per child during the study period. Several secondary outcomes will be documented using biological samples and validated questionnaires to ascertain laboratory-confirmed respiratory infections, intensity and severity of exacerbations, mean number of ED visits, parents' functional status during exacerbations, de-intensification of preventive asthma therapy, cost effectiveness, and safety profile. A sample of 432 children (400+7,5% attrition) per arm will provide 80% power with a two-tailed alpha of 5% to detect a 25% relative reduction in the mean number of exacerbations requiring OCS per child. An intention-to-treat (ITT) analysis will be carried out with all randomised children.