ArticlesWorldwide time trends in the prevalence of symptoms of asthma, allergic rhinoconjunctivitis, and eczema in childhood: ISAAC Phases One and Three repeat multicountry cross-sectional surveys
Introduction
The International Study of Asthma and Allergies in Childhood (ISAAC) epidemiological research programme was established in 1991 because of concern that asthma and allergies were increasing in prevalence and severity, but little was known about the scale of the problem worldwide or the factors affecting prevalence.1 Until the 1990s, most studies of the prevalence of asthma and allergies had been undertaken in the UK, Australia, and New Zealand. The ISAAC investigators believed that new information would be contributed by the participation of other countries, including developing countries, with comparisons between, rather than within populations, helped by standardised methods.
The enormous participation in ISAAC Phase One, in which 700 000 children from 156 centres in 56 countries were included, demonstrated the worldwide concern about asthma and allergies. The participatory ISAAC approach with simple questionnaires enabled the collection of comparable data from children throughout the world.2 The large variations in the worldwide prevalence of symptoms of asthma, allergic rhinoconjunctivitis, and eczema that were recorded, even in genetically similar groups,3, 4, 5, 6 suggested that environmental factors underlie the variations. Many aspects of environments have been examined in ecological analyses of data from ISAAC Phase One,7, 8, 9, 10, 11, 12, 13, 14 and have provided some support for hypotheses that economic development,15 dietary factors,7, 8 climate,9 infections,10 and pollens,11 might influence some of this variation.
In ISAAC Phase Two, causes are studied in more detail in 30 study centres in 22 countries, with detailed questionnaires and objective measurements of physiological variables and indoor exposure.16 From the outset, ISAAC Phase Three was planned to assess time trends in the prevalence of symptoms by repeating the original cross-sectional study after at least 5 years. Our aim was to examine the hypothesis that the prevalence of asthma, allergic rhinoconjunctivitis, and eczema is increasing in some, but not all, regions of the world. The findings might give further clues about the causes of these conditions by revealing information about geographical variation in the rate of change in symptom prevalence for the three disorders.
Section snippets
Methods
ISAAC Phase Three is a repetition of a multicountry cross-sectional survey of two age-groups of school children—6–7 years and 13–14 years—undertaken at least 5 years after the baseline survey, ISAAC Phase One. Phase One study participants were identified through random samples of schools in defined geographical areas, or by including all schools where the area had less than 3000 children of the age-group. In ISAAC Phase Three, the study centres chose the children either by new random samples of
Results
The ISAAC Phase Three studies were completed to the ISAAC standards in 172 data sets: 66 centres in 37 countries in the 6–7 year age-group (summarised in table 1 with full information in webtable 1) and in 106 centres from 56 countries in the 13–14 year age-group (summarised in table 1 with full information in webtable 2). Altogether, 190 data sets were received for both age-groups and 12 were excluded because of low response rates, four because of different sampling frames, and two because
Discussion
We have obtained worldwide comparable population estimates of direction and size of change in prevalence of symptoms of asthma, allergic rhinoconjunctivitis, and eczema, using identical instruments. For almost all centres, time trend data had not been previously recorded. This study included a large number of centres from around the world, and a large proportion of the centres participated in both Phase One and Phase Three, with about two-thirds of the original centres replicating the protocol
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