Objectives We investigated whether Scottish implementation of smoke-free legislation was associated with a reduction in unplanned hospitalisations or deaths (‘events’) due to respiratory tract infections (RTIs) among children.
Design Interrupted time series (ITS).
Setting/participants Children aged 0–12 years living in Scotland during 1996–2012.
Intervention National comprehensive smoke-free legislation (March 2006).
Main outcome measure Acute RTI events in the Scottish Morbidity Record-01 and/or National Records of Scotland Death Records.
Results 135 134 RTI events were observed over 155 million patient-months. In our prespecified negative binomial regression model accounting for underlying temporal trends, seasonality, sex, age group, region, urbanisation level, socioeconomic status and seven-valent pneumococcal vaccination status, smoke-free legislation was associated with an immediate rise in RTI events (incidence rate ratio (IRR)=1.24, 95% CI 1.20 to 1.28) and an additional gradual increase (IRR=1.05/year, 95% CI 1.05 to 1.06). Given this unanticipated finding, we conducted a number of post hoc exploratory analyses. Among these, automatic break point detection indicated that the rise in RTI events actually preceded the smoke-free law by 16 months. When accounting for this break point, smoke-free legislation was associated with a gradual decrease in acute RTI events: IRR=0.91/year, 95% CI 0.87 to 0.96.
Conclusions Our prespecified ITS approach suggested that implementation of smoke-free legislation in Scotland was associated with an increase in paediatric RTI events. We were concerned that this result, which contradicted published evidence, was spurious. The association was indeed reversed when accounting for an unanticipated antecedent break point in the temporal trend, suggesting that the legislation may in fact be protective. ITS analyses should be subjected to comprehensive robustness checks to assess consistency.
- priority/special populations
- public policy
- secondhand smoke
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Contributors JVB conceived the study, obtained funding, developed the methods, analysed the data, interpreted the findings and drafted the manuscript. DFM developed the methods, analysed the data, interpreted the findings and contributed to drafting the manuscript. CM, CPvS and JPP developed the methods, interpreted the findings and provided feedback on the manuscript. IS extracted data, interpreted the findings and provided feedback on the manuscript. AS conceived the study, obtained funding, developed the methods, interpreted the findings and supervised drafting of the manuscript.
Funding This work was funded by a Thrasher Research Fund Early Career Award (NR-0166) and the International Pediatric Research Foundation Young Investigators Exchange Programme. JVB is furthermore supported by fellowship grants from the Erasmus University Medical Centre and the Netherlands Lung Foundation (4.2.14.063JO). AS is supported by the Farr Institute. The funders had no role in study design, data collection and analysis, interpretation of the findings, decision to publish, or preparation of the manuscript.
Competing interests None declared.
Ethics approval National Health Service South East Scotland Research Ethics Service; The University of Edinburgh’s Centre for Population Health Sciences Ethics Review Group.
Provenance and peer review Not commissioned; externally peer reviewed.
Data sharing statement Those interested in accessing the data are advised to contact eDRIS.