Childhood asthma is a common disease in both high-income and lower-income countries. It is relatively more severe and increasing in prevalence in many lower-income countries.


It used to be thought that asthma primarily affected people in high-income countries. However, by the 1980s, several research groups had become concerned about signs that asthma was increasing in high-income countries, and that it was possibly much more common in low-income countries than initially presumed.

The resulting 20-year international study has shown that childhood asthma is a common disease in both high-income and lower-income countries. It is relatively more severe and increasing in prevalence in many lower-income countries.

ISAAC is established

Concern that asthma, rhinitis and eczema were on the increase led to a unique worldwide epidemiological research programme – the International Study of Asthma and Allergies in Childhood (ISAAC). Established in 1991 and coordinated from New Zealand and Germany, ISAAC has conducted research over a 20-year period. For many of the countries involved, the ISAAC research was their first-ever population based assessment of the prevalence and severity of asthma among children. ISAAC data offers the most comprehensive and up-to-date information available.

How ISAAC collected its data

ISAAC Phase One (1993–1997) and ISAAC Phase Three (2000–2003) were multi-centre, multi-country cross-sectional studies involving two age groups of school children: 13–14 year olds (adolescents) and 6–7 year olds (children).

Schools were randomly selected for participation from a defined geographical area. Written questionnaires on symptoms of asthma, rhinitis and eczema (translated from English, where necessary) were completed at school by the adolescents, and at home by the parents of the children. An asthma symptoms video questionnaire was also available for the adolescents. A sample size of 3,000 per age group was used to give sufficient confidence in the results from each area, and a high participation rate was required.

ISAAC Phase Two (1998–2004) conducted more intensive studies including clinical tests involving children aged 10–12 years old and was designed to investigate the relative importance of hypotheses of interest that arose from the Phase One results.

Which parts of the world participated in ISAAC?

ISAAC was the first truly global study of asthma and allergies in children. In Phase One adolescents and children from 165 centres in 62 countries took part. ISAAC Phase Two involved children in 30 centres in 22 countries. In Phase Three adolescents and children from 237 centres in 98 countries took part. In all, 306 centres in 105 countries participated in one or more phases of ISAAC (Figure 1). The countries were from all the regions of the world, representing 86.9% of the world’s population. While most countries had more than one centre, findings should not be assumed to represent the situation countrywide. In particular, most ISAAC centres were urban, so the findings are less likely to be representative of rural locations.

The 113 countries that did not participate in ISAAC represent only 13.1 % of the world’s population.

What ISAAC found

Asthma is a disease of low- and middle-income, as well as high-income, countries

ISAAC Phase One found that asthma symptoms (wheeze in the past 12 months) occurred in all countries studied. Although asthma symptoms were more common in some high-income countries, some low- and middle-income countries also had high levels of asthma symptom prevalence. In addition, there were striking variations in the prevalence of asthma symptoms throughout the world – up to 15-fold between countries. In Phase Three, it became clearer that a high prevalence of asthma symptoms is not restricted to high-income countries (Figures 2 and 3).

Asthma is more severe in low- and middle-income countries

In ISAAC Phase Three, the highest prevalence of symptoms of severe asthma* among participants with current wheeze was found in low- and middle-income countries, not high-income countries.

Asthma is on the increase in many countries

Research from English-language countries published in the 1990s reported increases in asthma prevalence from the 1980s, and continuing increases in prevalence had therefore been expected. However, ISAAC found that in most high-prevalence countries, particularly the English language countries, the prevalence of asthma symptoms changed little between Phase One and Phase Three (1993–2003), and even declined in some cases as shown in Figures 4 and 5.

In contrast, a number of countries that had high or intermediate levels of symptom prevalence in Phase One showed significant increases in prevalence in Phase Three. Examples include Latin American countries, such as Costa Rica, Panama, Mexico, Argentina and Chile, and Eastern European countries, such as the Ukraine and Romania. Most of the countries with very low symptom prevalence rates in Phase One reported increases in prevalence in Phase Three.

The overall percentage of children and adolescents reported to have ever had asthma increased significantly, possibly reflecting greater awareness of this condition and/or changes in diagnostic practice.

Asthma is increasing in many low- and middle-income countries

The ISAAC Phase Three time trends analysis showed that while asthma has become more common in some high-prevalence centres in high-income countries, in many cases the prevalence stayed the same or even decreased. At the same time, many low- and middle- income countries with large populations showed an increase in prevalence, suggesting that the overall world burden is increasing, and global disparities are decreasing.

What has happened to the prevalence and severity of asthma since 2003?

Funding has not yet been found for a further phase of ISAAC and there have been no other comparable standardised studies with wide international participation. It is therefore not possible to estimate whether the prevalences and severity of asthma have changed since 2003.

- Innes Asher, Tadd Clayton

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*See Appendix: Table 1 for definition