This paper details the prevalence of respiratory outcomes in a cohort of children who responded to both phases of the MANCAS study. The response rate for both studies was lower than anticipated, particularly so for MANCAS2. However, a publication from our centre8
reports that response to postal questionnaires has been falling in recent years and suggests that survey fatigue and reluctance to reveal personal information as possible reasons. In contrast to MANCAS2 where return of the questionnaire signified consent to participate potential participants in MANCAS1 were sent a letter describing the study and asking parents to return a refusal slip if they did not wish to participate. A study in Sweden9
which compared different approaches for consent in postal questionnaire surveys reported a higher response for the approach used in MANCAS1 when compared with the approach used for MANCAS2. The modification in the method of obtaining consent between the two MANCAS studies was necessary in order to comply with research ethics committee guidelines but might account for some of the differences in the response rate between the studies.
Missing data imputation was not considered an appropriate measure for addressing the low response rate as it was not possible to derive an accurate model due to the large amount of missing data.
Because of the low response rates the MANCAS studies are at increased risk of bias if those with respiratory symptoms were more likely to respond than those without such symptoms. The significantly lower prevalence of wheeze in MANCAS2 suggests that this was not the case. Furthermore, a comparison of the prevalence reported in the MANCAS studies with the prevalence reported in other studies of respiratory symptoms in children in the UK does not show an overwhelming preponderance of symptoms in the MANCAS studies, suggesting that responder bias is unlikely to have unduly skewed the prevalence identified in MANCAS. Using a different cohort of children in Manchester, a study carried out in the same centre reported a 12-month prevalence of 22% for wheeze for children aged up to 16 years in 2001 while the prevalence of night cough was 25%, and 14% reported using asthma medication in the past 12 months.5
An ISAAC written questionnaire study on the Isle of Wight identified 18.9% prevalence of current wheeze among 10 years olds10
and Malik et al.11
reported a prevalence of wheeze in the past 3 years of 22.2% for 9–12 year olds in Aberdeen.
The prevalence in MANCAS2 is derived from parental responses to questions rather than from the participants themselves. Possibly the prevalence for each of the outcomes measured would have been different if the adolescents themselves had completed the questionnaire but parental completion was retained to streamline, as much as possible, the methods employed for both MANCAS studies. A study comparing parental and self-reported responses to an ISAAC asthma questionnaire reveals higher current symptom prevalence reported by the adolescents than by the parents, more particularly so for wheeze after exercise.12
If a similar discrepancy occurred in our study then it is possible that significant differences in the prevalence would have been observed between the two studies. However, parents were encouraged to discuss the study with their child and to obtain their child's agreement before participating in the study. This is likely to have increased the possibility of both the child and parent completing the questionnaire together, a factor noted by Braun-Fahrlander et al12
to increase conformity between parental and adolescent response. A more recent study reported no significant differences between parent and adolescent responses to an ISAAC-based asthma questionnaire in a cohort of 13–14 year olds in a longitudinal asthma and allergy study.13
Gender differences in the prevalence of wheeze disappeared between the two time points supporting data from other studies that report gender shifts in the prevalence of asthma during puberty with higher prevalence in male children, similar prevalence during puberty and higher female prevalence in adulthood.14–17
Despite the gender shift there was no difference in the prevalence between the two time points hinting perhaps at an ever changing pattern of clinical manifestation of respiratory symptoms that cannot be unravelled from point prevalence data. Strachan et al17
report on a complex and changing pattern of respiratory symptom prevalence in a cohort followed from birth where various patterns of remission and relapse are observed over a 33-year period. At least two-thirds of the children in Strachan's study who wheezed at age 16 years had first wheezed before the age of 5 years. The Tucson Children's Respiratory Study showed that wheezing patterns are established by the age of 6 years and do not change up to age 16 years for children with respiratory symptoms in the preschool years.18
In the cohort who responded to both MANCAS studies there was no change in the prevalence of wheeze, wheeze with exercise or use of asthma medication over a 7-year period suggesting that, for this cohort, those who wheezed in early childhood continued to do so in adolescence, although the gender reconciliation indicates a shifting pattern within the cohort.
The significant decrease in antibiotic use for respiratory infection might suggest a decrease in respiratory infections suffered by children as they age or alternatively might indicate a growing reluctance among GPs (prescribers) to use antibiotics as a first-line treatment option.19
The significant decrease in night cough might signal maturation of the cough reflex pathway20
but also corresponds with adult data describing medically driven falls in adult nocturnal symptoms.21
The increased lifetime prevalence of hay fever/eczema might simply reflect the additional time available for these disorders to manifest in adolescence. Increases in hay fever and eczema in children in recent decades have also been reported in other studies.23