A nationwide study of asthma incidence rate and its determinants in Swedish pre-school children.

While many studies on asthma prevalence have been published, the number of studies on asthma incidence in pre-school children is limited. In this project, a nationwide sample of pre-school children was followed with the aim of estimating cumulative 5-year asthma incidence and its determinants. Parents of 5,886 children 1-6 years of age, sampled from day-care centres in 62 municipalities all over Sweden, responded in 2002 to a baseline postal questionnaire with questions regarding symptoms indicating possible asthma or atopic conditions, and a number of other variables. In 2007, parents of 4,255 children responded to an almost identical follow-up questionnaire. Of these, the 3,715 children who were free from asthma at baseline constitute the study population for this report. A large number of potential baseline determinants for cumulative 5-year asthma incidence were identified. Of these, food allergy, rhinitis, incomplete asthma diagnosis criteria (wheezing last 12 months, and ever had asthma but no current symptoms), parental rhinitis, parental asthma, age, and eczema, in ranking order of importance, remained significant in multivariate logistic regression analysis. The asthma incidence rate was highly dependent on presence or absence of these variables, the average annual rate ranging from 2/1,000/year in 6-year-olds with no determinants to 154/1,000/year in 1-year-olds with all determinants, corresponding to 11/1,000/year based on the whole study population.