Results

Of the 57 schools approached to participate, six in the purpose selected stratum declined and one in the randomly selected stratum declined. The latter school was replaced with a geographically neighbouring one, resulting in a study group of 51 schools. The questionnaire was distributed to an estimated 13 020 students and 9794 (75%) useable responses were obtained (85% of which were from the purpose selected group), excluding subjects who had a missing date or country of birth or both wheeze and wheeze frequency questions. Subjects were aged between 13 and 19 years; predominantly 14 (46.2%) and 15 (38.2%) years old, and 4569 (46.8%) subjects were boys. No significant difference was found in the 12 month period prevalence of reported wheeze among Australian born subjects in the purpose selected and randomly selected strata (1028 of 5428 (18.9%) and 254 of 1254 (20.3%); p = 0.3, respectively). For all subsequent analyses, the two strata have been combined. Table 1 displays the distribution of subjects and the 12 month period prevalence of wheeze by region of birth.

SYMPTOMS

The weighted 12 month period prevalence of wheeze or asthma attacks for the whole population was 18.9% (95% CI, 18.0 to 19.9). There were 9735 subjects who replied to the frequency of wheeze attacks question: 9.2% (95% CI, 8.7 to 9.8) reported between one and three wheeze attacks; 3.9% (95% CI, 3.6 to 4.3) between four and 12 attacks; and 1.9% (95% CI, 1.6 to 2.2) > 12 attacks each year. There were no significant differences in wheeze prevalence between sex or age groups.

SYMPTOMS AND REGION OF BIRTH

Table 1 shows the 12 month period prevalence of wheeze by region of birth. The prevalence for Australian born subjects (19.2%; 95% CI, 18.2 to 20.2) was more than twice that among those born outside Australia (8.5%; 95% CI, 7.4 to 9.7). There was no significant difference in the frequency distribution of wheeze attacks between the two groups (p = 0.22).

SYMPTOMS AND TIME LIVING IN AUSTRALIA

There was a strong trend to a higher 12 month prevalence of wheeze with longer time spent living in Australia (p < 0.001) (table 2). This effect was unaltered by adjustment for age and region of birth.

Discussion

Different methodology and definitions have hampered the epidemiological study of asthma.12 The international study of asthma and allergies in childhood (ISAAC)13 has standardised the methodology and a video questionnaire may help with problems of translation and language.14 Our study used the same questions as a 1990 Melbourne study,2 which reported a 12 month period prevalence of wheezing or asthma attacks ranging from 23.1% for 7 year olds to 18.6% for 15 year olds. Our data do not indicate an increase in the 12 month period prevalence of wheeze over the past five years and they do not support the suggestion that the prevalence of respiratory symptoms continues to increase, at least in Melbourne. Aberdeen data, using a three year period prevalence of wheeze, reported a significant increase between 1989 and 1994, from 19.8% to 25.4% in 8–13 year olds.6 Comparing these studies illustrates the difficulties of using two different period prevalences as outcome measures. Another study of Australian children aged 13–14 years in 1994–5 found a 12 month period prevalence of wheeze of 29.4% (95% CI, 29.1 to 29.7),15 and these data support the theory of the prevalence increasing. However, the latter study used the ISAAC questions, which were slightly different from the questions used in 1990. Furthermore, in the 1990 study the parents completed the questionnaire, whereas the subjects completed the questionnaire themselves in the larger 1995 ISAAC study. This may be an important factor because there are differences between the rates of self reported and parent reported symptoms.16 The overall evidence for an increase in prevalence of asthma has been considered weak because the measures used to estimate the prevalence of symptoms are susceptible to systematic errors.5 Thus, it is imperative that the same questionnaires and mode of completion are used when comparing repeated studies.

Selection and participation biases might have affected the estimated prevalences. Only 15% of respondents were randomly selected, but the total sample was drawn from almost one third of Melbourne schools. The 85% of respondents from schools purposely selected because of their relatively high numbers of overseas born students would not be expected to have different patterns of asthma from those in other schools. This reasonable assumption was supported by our data, which showed no difference in asthma prevalence among Australian born students between the two groups of schools (random and purpose selected). Compared with other studies,2 3 we obtained a lower questionnaire response rate at 75%. Although there are no data to confirm this, it seems unlikely that the likelihood of participation would be strongly related to the presence of wheezing history; however, any such bias might be expected to result in an underestimation of the prevalence of symptoms and the level of treatment. Estimates of population prevalence were standardised to the appropriate distribution of region of birth, thus compensating for the major known bias in sample selection. We did not collect data on smoking and the change in smoking habits over time; some might argue that smoking (active or passive) may be a confounding factor on our conclusions.

The standardised methodology of ISAAC has allowed, for the first time, a comparison of the prevalence of respiratory symptoms from different geographical areas.17 Wide variations of symptom prevalence were found: a 12 month period prevalence of wheeze ranging from 1.8% to 35.3%, with a mean of 14.2% for 13–14 year olds.17 Our data also reveal different prevalences of recent wheeze based on region of birth. The prevalence of wheeze was highest in Australian born subjects and much lower in subjects who were born outside Australia. Making comparisons with studies completed in those countries is difficult because of the methodological problems already alluded to. However, there does appear to be a difference in wheeze prevalence between children from “Westernised countries” and developing countries.18

A striking finding of our study is that duration of residence in Australia was strongly associated with reported symptoms. There is a “dose” related response; the longer time spent living in Australia, the more likely the population will adopt the prevalence of the local born population. When controlled for child’s age and region of birth the effect remained. This supports the notion that the environment continues to be an important factor in influencing the prevalence of respiratory symptoms even after the early years of sensitisation.8-10 In a contemporaneous study in four Australian states, Robertson et aldemonstrated that Australian born subjects were more likely to report current wheeze when compared with those born elsewhere15; our data support their findings but add further evidence of a dose time effect. Our study also found that although the prevalence of wheezing varied depending on region of birth, the severity and frequency of attacks were not significantly different between Australian born and non-Australian born subjects. There were no significant differences in the prevalence of symptoms between boys and girls. This peripubertal sex equilibration of the prevalence of wheeze is well known.2 19

Studies using “race” and “ethnicity” as outcome or predicting variables11 have problems because of the inherent difficulties of defining those terms.20 21 We have used “region of birth” as a variable, but this could be criticised because there is evidence of differences between two races from the same geographical region.22 23 It has been suggested that differences in wheeze prevalence in different populations must depend primarily on differences in inherited or acquired hyper-responsiveness and only secondarily on differences in their exposure to environmental factors capable of provoking it.24 The dose–time effect that we have demonstrated in subjects living in Melbourne, which is independent of region of birth, would reinforce the strong influence of an environmental factor. Whether this is the “Westernisation factor” suggested by Gregg24 and others7 12 cannot be answered by a descriptive study such as ours.

Studies examining the prevalence of respiratory symptoms in migrating populations have yielded conflicting results. In South Africa, Xhosan children living in Transkei had a much lower incidence of exercise induced symptoms compared with children who had moved from Transkei to townships near Capetown.25 Leung et al demonstrated large differences between Asian immigrants, Australian born Asians, and non-Asians, with higher levels of reported atopic symptoms in subjects who had been born in Australia compared with those who were born outside the country,26 and with similar dose–time effects to those found in our study. Tokelauan children who migrated to New Zealand had twice as many asthma symptoms as those who stayed on the island.27 Asian children born in Africa have a reported similar prevalence of asthma symptoms to children born in the UK, and it has been postulated that this may be because the lifestyle in Africa is more “Westernised” than it is in the Indian subcontinent.28 Children born in Pakistan or India and moving to the UK reported lower symptom rates compared with children born in the UK.29 An increase in reported asthma symptoms in Papua New Guinea between 1972 and 198030 31was considered to be caused by the introduction of blankets and the house dust mite into the community.32 Our data are consistent with the existence of an environmental factor or factors affecting the prevalence of symptoms, independently of genetic susceptibility.