Study population
The CHILD Cohort Study is a Canadian birth cohort consisting of 3455 children and their parents.(18,19) Mothers were initially recruited during the second or third trimesters of pregnancy from four provinces across Canada: British Columbia (Vancouver), Alberta (Edmonton), Manitoba (Winnipeg, Morden and Winkler), and Ontario (Toronto). Their children have been followed from birth onwards, with a home visit at 3-4 months of age, repeated questionnaire surveys examining environmental exposures, and clinic visits for detailed assessments and obtaining biospecimens (e.g., blood, stool, urine, nasal swabs) at ages 1, 3, and 5 years.(19–21) Written informed consent was obtained from the parents of children enrolled in this study. Protocols were approved by the Human Research Ethics Boards at Queen’s University, McMaster University, the Hospital for Sick Children, and the Universities of Manitoba, Alberta, and British Columbia.