Statistical analysis
The data were analyzed using IBM SPSS Statistics for Windows, version 25.0 software (IBM Corp., NY, U.S.A.). Descriptive statistics are presented as numbers and proportions or medians, with 25–75% interquartile ranges (IQRs). The Mann–Whitney U -test was used for comparisons of continuous variables. Logistic regression was used for both univariate and multivariate analyses of categorical variables, and the results were presented as odds ratios (ORs), with their 95% confidence intervals (95% CIs). Multivariate models for the risk of current asthma were adjusted for current and early-life confounding factors, when appropriate. The current confounders were sex and current daily smoking. The early confounders were age at the time of index hospital admission (< 12 or 12 months), presence of household pets, or presence of pets in day care and passive smoke exposure during infancy.