Study design and study sample
The Swedish Twin Study on Prediction and Prevention of Asthma (STOPPA) is a population-based, cross-sectional twin study on childhood asthma in discordant, concordant, and healthy concordant twins22. The study participants were recruited from the Child and Adolescent Twin Study in Sweden (CATSS) 23, a study initiated in 2004 that included all twins born from July 1992 and onwards, identified through the Swedish Twin Registry24. Validated questions on asthma ever (yes/no) and wheezing (current or after three years of age) were used to identify 9- to 14-year-old twins discordant and concordant for asthma or wheeze and healthy control twins according to the International Study of Asthma and Allergies in Childhood (ISAAC) 25. Monozygotic (MZ) and same-sex dizygotic (DZ) twin pairs who were raised together were recruited to STOPPA from all parts of Sweden and distributed throughout the whole year. The study population (n = 752) was classified as asthma concordant (31%), asthma discordant (38%), and healthy concordant (31%), according to the recruitment algorithm of asthma status, as described in detail elsewhere 22. One twin pair had unknown zygosity, and 138 individuals had missing data on inhaled corticosteroid steroid (ICS) use. The final analytic sample consisted of 612 individuals (81.4% of the study population with full information on all covariates). The study was approved by the Regional Ethical review board in Stockholm, Sweden. Informed consent was obtained from all the participants and their parents.