Asthma is the commonest chronic disease in children and adolescents globally with about 10% of children and adolescents globally having had asthma symptoms in the last year. The Global Asthma Network (GAN) has continued the work of the International Study of Asthma and Allergies in Childhood (ISAAC), enabling us to assess global time trends in childhood asthma.
Asthma symptoms were assessed from centres which completed GAN Phase I and ISAAC Phase I (1993-5) and/or III (2001-3). There were 119,795 participants from 27 centres in 14 countries. The prevalence of current wheeze decreased in low-income countries (6-7 year-olds: -1·37, 95% CI [-2·47,-0·27], 13-14 year-olds: -1·67, 95% CI [-2·70,-0·64]), increased in lower-middle-income countries (6-7 year-olds: 1·99, 95% CI [0·33, 3·66], 13-14 year-olds: 1·69, 95% CI [0·13, 3·25]) but was stable in upper-middle and higher income countries.
The trends in asthma prevalence and severity over the last three decades are not uniform, and vary by age group, country income, region and centre. The reasons for these trends are still unclear. In general, asthma prevalence increases as countries become more affluent and “Western”. However, these trends are not uniform, and are not explained by “established” asthma risk factors such as allergen exposure, smoking or air pollution. The “hygiene hypothesis” may partly explain the time trends, but there are some important anomalies, with some countries such as Brazil showing high prevalence, even though they have high infection rates. Decreasing biodiversity may also play a role.