Childhood respiratory cohort studies: Do they generate useful outcomes?

Cohort studies give insight into the evolution of respiratory disease over time and provide a low level of evidence for causation. The initial pioneering cohort studies in which symptoms and/or exposures were captured in early life and related to respiratory outcome in later life are now part of a large family which is diverse in age and exposures; however, asthma remains a fairly constant outcome. This article aims to first describe the strengths and weaknesses of the various asthma cohort studies. Thereafter, the article aims to describe the insight that cohort studies have given into asthma causation, with a focus on relative magnitude of effect, and also to describe the relative weight of factors identified in cohort studies as predictors of persisting asthma symptoms. It is clear that many factors are implicated in asthma causation; in isolation, each exerts a rather modest positive of negative effect on causation, but are likely to be acting in combination and modifying each other's relative risk. There is no single "good" predictor of asthma outcome, but the coexistence of parental asthma/ atopy in a child with at least three episodes of wheeze by their third birthday indicates a high relative risk for asthma at 11-13 years of age. There remains no cure for asthma and the next step needs to apply what cohort studies have taught us to intervention studies where several exposures are modified in at risk individuals during the first year of life.