The magnitude of the effect of smaller family sizes on the increase in the prevalence of asthma and hay fever in the United Kingdom and New Zealand

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1 The magnitude of the effect of smaller family sizes on the increase in the prevalence of asthma and hay fever in the United Kingdom and New Zealand Kristin Wickens, DPH, Julian Crane, FRACP, Neil Pearce, PhD, and Richard Beasley, MD Wellington, New Zealand Background: Declining family size is one factor that has been proposed to contribute to increasing asthma and hay fever prevalence, but its relative importance has not been quantified. Objective: Our purpose was to determine the change in asthma and hay fever prevalence that would be expected from the reduction in family size that has occurred in England/Wales and New Zealand over recent decades. Methods: The relative change in family size between 1961 and 1991 in England/Wales and New Zealand was determined from census data for these years. Summary weighted odds ratios were calculated for the associations among birth order, family size, and asthma and hay fever prevalence. The expected increase in the prevalence of asthma and hay fever between 1961 and 1991 resulting from changes in family size was then calculated. Results: The expected relative increase in the prevalence of asthma between 1961 and 1991 as a result of the smaller family size was 1% and 5% for England/Wales and New Zealand, respectively; smaller family size would be expected to increase the prevalence of hay fever prevalence in England/Wales by 4%. Conclusions: Changes in family size over the last 30 years do not appear to explain much of the reported increase in asthma or hay fever prevalence. The contribution that other risk factors have made to these increases could be assessed with use of a similar approach. (J Allergy Clin Immunol 1999;104:554-8.) Key words: Asthma prevalence, hay fever prevalence, family size, birth order, England, Wales, New Zealand There has been a reported increase in the prevalence of symptoms of asthma 1-7 and hay fever 3,8 in many countries, including the United Kingdom and New Zealand, over recent decades. Although the possibility of changes in recognition and reporting of symptoms in the population-based samples has limited the interpretation of these observations, 9 they are nevertheless likely to, at least partly, reflect a true increase in the prevalence of asthma From the Wellington Asthma Research Group, Department of Medicine, Wellington School of Medicine, Wellington, New Zealand. Supported by a Programme Grant from the Health Research Council of New Zealand. Received for publication Feb 2, 1999; revised Apr 21, 1999; accepted for publication May 14, 1999 Reprint requests: Kristin Wickens, DPH, Wellington Asthma Research Group, Wellington School of Medicine, PO Box 7343, Wellington South, New Zealand. Copyright 1999 by Mosby, Inc /99 $ /1/99998 and hay fever in both children and adults. A number of hypotheses have been put forward to explain these trends, including reduced frequency of childhood infections, 10 widespread effective immunization programs, 10 reduction in the prevalence of tuberculosis, 11 increased antibiotic use, 12 changes in diet, 13 factors associated with enhanced fetal growth, 14 increased exposure to indoor allergens such as the house dust mite, 15 increased exposure to passive smoking, 16 increased outdoor air pollution, 17 reduced parasite load, 14 changes in lifestyle including less exercise, 15 and smaller family size. 18 It has been difficult to determine not only whether these factors may actually contribute to the risk of development of allergic disease but also their relative importance in terms of the magnitude of the effect they may have had on the increasing prevalence of asthma and hay fever that has been observed over recent decades. Because the magnitude of the association of family size with asthma or hay fever risk has been estimated in several studies and the necessary demographic data are available from government statistics, family size is one factor that can be assessed in terms of estimating its effect on the changing prevalence of allergic disease. In this study we calculated the change in asthma prevalence that would be expected from the reduction in family size that has occurred in England/Wales and New Zealand over the last 3 decades. We have also calculated the change in hay fever prevalence expected as a result of smaller family size in England/Wales. Published studies were not available to calculate the associations between family size and hay fever in New Zealand. METHODS Data on family size for England/Wales and New Zealand was obtained from the Office of Population Censuses and Surveys 19 and Statistics New Zealand, 20,21 respectively, for the period 1961 to For the United Kingdom data were available for the birth order of children, expressed as a percentage of children. For New Zealand data were available for the number of children per family, expressed as a percentage, based on all families with children. The relative change in family size over the 3-decade period was determined by comparison of the data in 1961 and We searched the MEDLINE database for the years (November) and the Current Contents database for the years (November) for published studies reporting the effect of family size or birth order on the development of asthma. In MEDLINE we searched the medical subject headings asthma or wheeze 554

2 J ALLERGY CLIN IMMUNOL VOLUME 104, NUMBER 3, PART 1 Wickens et al 555 TABLE I. Characteristics of studies included in analysis Study Country Study design Population (y) Sample size Outcome measure Leadbitter et al 33 New Zealand Cohort Asthma ever Wickens et al 34 New Zealand Case-control cases/241 controls Diagnosed asthma and current medication use Shaw et al 35 New Zealand Cross-sectional Current wheeze Moyes et al 32 New Zealand Cross-sectional 6-7 2,614 Diagnosed asthma Lewis et al 36 England, Scotland, Cohort 16 20,528 Current asthma and and Wales or wheezy breathing Jarvis et al 24 England Cross-sectional ,159 Asthma attacks Hay fever/nasal allergy Rona et al 37 England and Scotland Cross-sectional ,924 Wheezing or asthma attacks Bodner et al 38 Scotland Cross-sectional ,111 Diagnosed asthma ever Hay fever ever Farooqi and Hopkin 39 England Cohort ,934 Diagnosed asthma Diagnosed hay fever Butland et al 8 England, Wales, and Scotland Prospective 16 11,195 (1958 Hay fever or allergic cohort), 9,387 rhinitis (1970 cohort) Strachan et al 40 England Cohort ,765 Diagnosed hay fever TABLE II. Birth order of children in England/Wales Birth order (No. of older siblings) Year First (0) Second (1) Third (2) Fourth+ (3+) Total % 30.5% 16.4% 16.4% 100% % 36.7% 15.6% 8.2% 100% Odds ratio (95% confidence interval) ( ) 0.81 ( ) 0.87 ( ) for asthma Odds ratio (95% confidence interval) ( ) 0.65 ( ) 0.51 ( ) for hay fever TABLE III. Number of children per family based on all families with children in New Zealand Family size (No. of siblings) Year 1 (0) 2 (1) 3 (2) 4 (3+) Total % 30.9% 20.9% 20.6% 100% % 38.5% 18.2% 7.3% 100% Odds ratio (95% confidence interval) ( ) 0.71 ( ) 0.56 ( ) for asthma and combined these with family size, sibling, or birth order ; in Current Contents we used the same strategy but searched abstracts, titles, keywords, and keywords plus. In total, 14 studies were identified on the basis of populations in either the United Kingdom or New Zealand in which the prevalence odds ratio for asthma in association with either birth order or family size was reported or the information needed for this calculation was available. Of the 14 studies reviewed, 2 were excluded because the participants included infants and there are problems with defining asthma in this population, and 3 studies were excluded because the information needed to calculate a summary odds ratio across studies was not reported. Combining the data from the remaining 9 studies, we calculated the summary odds ratio, weighted by the inverse variance of the estimates in each study, 22 for each subgroup of family size and birth order. For hay fever we used the same approach but searched on hay fever or rhinitis instead of asthma or wheeze. We found 5 studies that showed associations between hay fever and birth order in the United Kingdom. Because only 2 United Kingdom studies were located that reported sufficient data for calculation of summary odds ratio for the association between family size and hay fever, only studies reporting associations between birth order and hay fever were included in the analysis. No New Zealand studies were located that reported associations between family size variables and hay fever. The expected increase in the prevalence of asthma and hay fever resulting from changes in family size in England/Wales was calculated by weighting the proportion in each birth order category for 1991 and 1961 by the summary odds ratio for that category. The relative risks for having asthma and hay fever in 1991 compared with 1961 were then calculated. A similar calculation was performed to estimate the expected increase in asthma attributable to changes in family size in New Zealand.

3 556 Wickens et al J ALLERGY CLIN IMMUNOL SEPTEMBER 1999 FIG 1. Weighted summary odds ratios for having asthma by number of older siblings. FIG 2. Weighted summary odds ratios for having asthma by total number of siblings. RESULTS FIG 3. Weighted summary odds ratios for having hay fever by number of older siblings. The studies included in the analysis are shown in Table I. The summary odds ratios for asthma in association with birth order and number of children in the family are shown in Figs 1 and 2, respectively; summary odds ratios for the association between hay fever and birth order are shown in Fig 3. Although there is a marked trend toward reduced asthma risk with declining family size (Fig 2), the risk of asthma was reduced for second-born children compared with first-born children but was similar for all later-born children (Fig 1). There was also a trend toward a lower prevalence of hay fever with increasing birth order. In both England/Wales and New Zealand the mean number of children per family progressively decreased between 1961 and In England and Wales the proportion of children with a birth order of 4 or more was 8.2% in 1991 compared with 16.4% in 1961 (Table II). In New Zealand the proportion of families with 4 or more children was 7.3% in 1991 compared with 20.6% in 1961 (Table III). With use of the methods outlined above, we calculated the relative risk for having asthma in 1991 compared with 1961 and showed that the expected relative increase in the prevalence of asthma between 1961 and 1991 resulting from the smaller family size was 1% and 5% for England/Wales and New Zealand, respectively. The expected relative increase in the prevalence of hay fever between these years because of smaller family size was 4%.

4 J ALLERGY CLIN IMMUNOL VOLUME 104, NUMBER 3, PART 1 Wickens et al 557 DISCUSSION Although most studies show an increase in asthma and hay fever prevalence over time, estimating the relative size of this increase between the years 1961 and 1991 (when government statistics were available on family size) is difficult because of the different dates at which point prevalences are reported. For example, no studies were located that reported asthma symptom or hay fever prevalence at 1961 and at However, a Scottish study 1 reported an increase in wheeze prevalence from 10.4% to 19.8% between 1964 and 1989 (a relative increase of 90%), and more recent data from New Zealand 2 reported that over a shorter and more recent time period ( ) the prevalence of asthma symptoms increased from 26.2% to 34.0% (a relative increase of 30%). Although most other United Kingdom and New Zealand studies report similar increases in asthma prevalence, a study by Butland et al 7 reported a small increase in asthma symptom prevalence from 11.1% to 12.9% between 1978 and 1991 (a relative increase of less than 2%). There are fewer studies reporting changes in hay fever prevalence in the United Kingdom (and none in New Zealand), but of the published studies the increase in hay fever appears to be consistently large. For example, in Scotland between 1964 and 1989 hay fever prevalence was reported 1 to increase from 3.2% to 11.9% (a relative increase of 372%). We found that the progressive reduction in family size between 1961 and 1991 (2- to 3-fold reduction in the proportion of families with 4 or more children) may have led to a 1% to 5% relative increase in the prevalence of asthma in childhood in the United Kingdom and New Zealand and to a 4% relative increase in the prevalence of hay fever in the United Kingdom. Therefore the reductions in family size in the last 30 years account for little of the reported increase in asthma 1-7 or hay fever. 1,3,8 Thus despite the strong associations between family size and asthma (odds ratio = 0.56 for children with 3 or more siblings compared with only children) and between birth order and hay fever (odds ratio = 0.51 for children with 3 or more older siblings compared with only children), the role that family size plays in explaining increases in asthma or hay fever prevalence is relatively minor. There have reports of similar inverse associations between family size and atopic sensitization. 23,24 However, although some reports suggest that atopy has increased over time, 25,26 such reports have been sparse and inconsistent. 27 Therefore we did not include an analysis of atopy in this article. It is recognized, however, that a substantial increase in atopy prevalence would contribute to increasing asthma prevalence; however, the size of this contribution remains unclear as atopy may account for less than one half of asthma cases in children and adults. 28 It should also be noted that the studies excluded from this review because the data were not available for calculation of a summary odds ratio 29,30 or the data were incomplete 31 reported no association between family size and asthma 29,30 or a positive association. 31 These exclusions together with possible publication bias means that the association between family size and asthma may have been overestimated; thus the contribution that family size makes toward asthma symptom prevalence may also have been overestimated. Although no articles reporting associations between birth order and hay fever were excluded, a possible publication bias may also have resulted in an overestimation of the contribution that family size makes toward hay fever prevalence. In conclusion, changes in family size over the last 30 years do not appear to explain much of the reported increase in asthma symptom or hay fever prevalence. We suggest that a similar approach is used to assess the relative importance of other risk factors that may have changed over time (eg, women smoking, dietary factors, antibiotic use) and may contribute to an increasing prevalence of allergic disease. We thank Dr Christopher D. Moyes for providing us with more detailed data than was reported in his publication. 32 REFERENCES 1. Ninan T. Respiratory symptoms and atopy in Aberdeen schoolchildren: evidence from two surveys 25 years apart. BMJ 1992;304: Shaw R, Crane J, O Donnell T, Porteous L, Coleman E. 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