Worldwide variation in prevalence of symptoms of asthma, allergic rhinoconjunctivitis, and atopic eczema: ISAAC. The International Study of Asthma and Allergies in Childhood (ISAAC) Steering Committee.
BACKGROUND
Systematic international comparisons of the prevalences of asthma and other allergic disorders in children are needed for better understanding of their global epidemiology, to generate new hypotheses, and to assess existing hypotheses of possible causes. We investigated worldwide prevalence of asthma, allergic rhinoconjunctivitis, and atopic eczema.


METHODS
We studied 463,801 children aged 13-14 years in 155 collaborating centres in 56 countries. Children self-reported, through one-page questionnaires, symptoms of these three atopic disorders. In 99 centres in 42 countries, a video asthma questionnaire was also used for 304,796 children.


RESULTS
We found differences of between 20-fold and 60-fold between centres in the prevalence of symptoms of asthma, allergic rhinoconjunctivitis, and atopic eczema, with four-fold to 12-fold variations between the 10th and 90th percentiles for the different disorders. For asthma symptoms, the highest 12-month prevalences were from centres in the UK, Australia, New Zealand, and Republic of Ireland, followed by most centres in North, Central, and South America; the lowest prevalences were from centres in several Eastern European countries, Indonesia, Greece, China, Taiwan, Uzbekistan, India, and Ethiopia. For allergic rhinoconjunctivitis, the centres with the highest prevalences were scattered across the world. The centres with the lowest prevalences were similar to those for asthma symptoms. For atopic eczema, the highest prevalences came from scattered centres, including some from Scandinavia and Africa that were not among centres with the highest asthma prevalences; the lowest prevalence rates of atopic eczema were similar in centres, as for asthma symptoms.


CONCLUSIONS
The variation in the prevalences of asthma, allergic rhinoconjunctivitis, and atopic-eczema symptoms is striking between different centres throughout the world. These findings will form the basis of further studies to investigate factors that potentially lead to these international patterns.
D000293:Adolescent; D001249:Asthma / Q000175:diagnosis / Q000453:epidemiology*; D003876:Dermatitis, Atopic / Q000175:diagnosis / Q000453:epidemiology*; D006801:Humans; D015995:Prevalence; D006255:Rhinitis, Allergic, Seasonal / Q000175:diagnosis / Q000453:epidemiology*; D011795:Surveys and Questionnaires