Elsevier

The Lancet

Volume 368, Issue 9537, 26 August–1 September 2006, Pages 733-743
The Lancet

Articles
Worldwide 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

https://doi.org/10.1016/S0140-6736(06)69283-0Get rights and content

Summary

Background

Data for trends in prevalence of asthma, allergic rhinoconjunctivitis, and eczema over time are scarce. We repeated the International Study of Asthma and Allergies in Childhood (ISAAC) at least 5 years after Phase One, to examine changes in the prevalence of symptoms of these disorders.

Methods

For the ISAAC Phase Three study, between 2002 and 2003, we did a cross-sectional questionnaire survey of 193 404 children aged 6–7 years from 66 centres in 37 countries, and 304 679 children aged 13–14 years from 106 centres in 56 countries, chosen from a random sample of schools in a defined geographical area.

Findings

Phase Three was completed a mean of 7 years after Phase One. Most centres showed a change in prevalence of 1 or more SE for at least one disorder, with increases being twice as common as decreases, and increases being more common in the 6–7 year age-group than in the 13–14 year age-group, and at most levels of mean prevalence. An exception was asthma symptoms in the older age-group, in which decreases were more common at high prevalence. For both age-groups, more centres showed increases in all three disorders more often than showing decreases, but most centres had mixed changes.

Interpretation

The rise in prevalence of symptoms in many centres is concerning, but the absence of increases in prevalence of asthma symptoms for centres with existing high prevalence in the older age-group is reassuring. The divergent trends in prevalence of symptoms of allergic diseases form the basis for further research into the causes of such disorders.

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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