METHODS
Population and study procedures
The details of this birth cohort (NCT02903134) have been published elsewhere7. Briefly, pregnant women were recruited during their stay in the delivery ward in Santiago, Chile, after consent was obtained. Information collected included parental sociodemographic, home characteristics, and perinatal characteristics of the offspring, as previously described7. Cord blood was collected, processed as previously described7, and stored for analysis. Children were followed by phone every 6 months through age 24 months, and in-person at age 30 months. At each visit information was collected about feeding characteristics, pets, second-hand tobacco smoking, acute respiratory illnesses, siblings, day care attendance, as well as physician diagnosis of asthma and other atopic diseases. Any diagnoses of asthma and their treatment was made independently by the participants’ primary care providers (PCPs), who were not part of the current study. The Edinburg scale for maternal depression postdelivery was performed at 6 months. The study was approved by the Ethics Committees of the participating institutions.
Our primary outcome was physician-diagnosed asthma during the first 30 months (made independently by the PCP not involved in the study). The main risk factor evaluated was a positive API (API+) according to the original stringent criteria: children had to be defined as an early frequent wheezer (more than 3 episodes) during the first 3 yr of life and meet at least one of two major criteria (parental MD asthma or MD eczema in the child) or two of three minor criteria (MD allergic rhinitis, wheezing apart from colds, or eosinophilia)5. The API was constructed at the end of this study by us, independently, and blinded to the main outcome of physician-diagnosis asthma made by the PCP not involved in the study.