Birth Cohort Studies on Asthma Development

In past decades the prevalence of asthma in children has increased substantially but this trend seems not to have persisted into the twenty-first century. What might have caused the increased prevalence is not well understood. With the expectation that the prevalence may be decreased by reducing exposure to environmental factors, several birth cohort studies are being (or have been) carried out all over the world. Although many studies are still ongoing, decisions on data gathered by these studies will be of great importance for clinical practice. An overview of designs of available studies will be essential in constructing a metaanalysis. We describe the similarities and differences between designs and variable sets of birth cohort studies focusing on the relationship between allergen exposure (solely or in combination with other environmental exposures) and development of childhood asthma. We also evaluate which studies are particularly qualified for inclusion in a meta-analysis relating asthma prevention to environmental exposures. Ongoing birth cohort studies on allergen exposure (solely or in combination with other environmental exposures) in relation to asthma development in children were identified and all available information on study designs was collected and compared. From the 19 studies selected, 13 are prospective cohort studies and 6 are randomized clinical trials (RCTs). All 6 RCTs included children during prenatal life whereas all but 5 prospective cohort studies started after birth. The exposures that almost all studies focus on are house dust mite (HDM) allergens, pet allergens, food allergens, and environmental tobacco smoke (ETS). We concluded that RCTs are the best candidates for inclusion in a meta-analysis (an individual subject data analysis) in the future. Four RCTs we evaluated as being particularly suitable for inclusion in a meta-analysis on the basis of similarities in their study designs: the Isle of Wight Study, the Canadian Allergy and Asthma Project (CAAP), the CAPS Study, and the PREVASC Project. Of these four all recommend multifaceted interventions, and differences in their designs are expected to be overcome by stratification and restriction to homogeneous clusters. (Pediatr Asthma Allergy Immunol 2005; 18[4]:201–215.)