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

Inclusion Criteria: * Age 1-5 years * Physician-diagnosed asthma (as per the 2015 Canadian Position Paper on the diagnosis of preschool asthma) * ≥1 asthma exacerbation requiring rescue oral corticosteroids (OCS) in the past 6 months or ≥2 in the past 12 months; or from the pandemic (2020) onwards, ≥1 asthma exacerbation requiring rescue oral corticosteroids (OCS) in the past 12 months (as documented by pharmacy/medical records) * ≥4 upper respiratory tract infections (URTIs) in the past 12 months (as per parental report); or from the pandemic (2020) onwards, ≥ 2 URTIs in past 12 months * URTIs as the main asthma trigger (as per parental report) Exclusion Criteria: * Intake \> 400 IU/day of vitamin D3 supplements or fish oil in the past 3 months * Intention to use \> 400 IU/day of vitamin D3 supplements or fish oil in the fall and winter * Extreme prematurity (\< 28 week gestation) * No vitamin D supplementation (if breast-fed in the last 6 months) * Vitamin D restrictive diets, that is, minimal intake of vitamin D fortified milk (\<250 mL/day for 1-3 years or \<375 mL/day for 4-6 years AND no other (or \<200 IU/day) vitamin D supplement * Recent immigrants from regions at high risk of rickets (in the past 12 months) * Recent refugees (in the past 12 months) * Undernourished children * Other chronic respiratory disease (e.g. Cystic fibrosis, Bronchopulmonary dysplasia) or chronic kidney, gastrointestinal, endocrinological or cardiac diseases, or sickle cell anemia * Histor…