Previous studies of maternal influenza illness and preterm birth have yielded inconsistent results. Our objective was to assess the association between 2009 pandemic H1N1 (pH1N1) influenza during pregnancy and preterm birth in a large obstetrical population.
We linked a province-wide birth registry with health administrative databases to identify influenza-coded hospitalizations, emergency department visits, or physician visits among pregnant women during the 2009 H1N1 pandemic (our proxy for clinical pH1N1 influenza illness). Using Cox regression, we estimated adjusted hazard ratios (aHR) for preterm birth and spontaneous preterm birth treating influenza as a time-varying exposure.
Among 192,082 women with a singleton live birth, 2,925 (1.5%) had an influenza-coded health care encounter during the 2009 H1N1 pandemic. Compared with unexposed pregnancy time, there was no association between exposure to the pandemic, with or without clinical influenza illness, and preterm birth (no pH1N1 diagnosis: aHR=1.0; 95% confidence interval [CI]: 0.98, 1.1; pH1N1 diagnosis: aHR=1.0; 95% CI: 0.88, 1.2). Among women with pre-existing medical conditions, influenza was associated with increased preterm birth (aHR=1.5; 95% CI: 1.1, 2.2) and spontaneous preterm birth (aHR=1.7; 95% CI: 1.1, 2.6), and these associations were strongest in the third trimester and when data were analyzed to allow for a transient acute effect of influenza.
In the general obstetrical population, there was no association between pH1N1 influenza illness and preterm birth, but women with pre-existing medical conditions known to increase the risk of influenza-associated morbidity were at elevated risk.
Conflicts of Interest: The authors have no conflicts to declare.
Source of Funding: This study was partially funded through an Operating Grant from the Canadian Institutes for Health Research (#MOP-119570). This study was supported by the Institute for Clinical Evaluative Sciences (ICES), which is funded by an annual grant from the Ontario Ministry of Health and Long-Term Care.
Data Access: The dataset from this study is held securely in coded form at the Institute for Clinical Evaluative Sciences (ICES). While data sharing agreements prohibit ICES from making the data set publicly available, access may be granted to those who meet pre-specified criteria for confidential access, available at www.ices.on.ca/DAS.
Acknowledgments: DBF was supported by a Canadian Institutes of Health Research (CIHR) Doctoral Award while this study was being conducted. RWP is supported by a Chercheur-national (National Scholar) award from the Fonds de Recherche du Quebec – Santé (FRQ-S) as well as core support to the Research Institute of the McGill University Health Centre from FRQ-S. JCK is supported by a New Investigator Award from CIHR and a Clinician Scientist Award from the University of Toronto Department of Family and Community Medicine. We are grateful to Better Outcomes Registry & Network (BORN) Ontario, Ottawa, Canada and the Institute for Clinical Evaluative Sciences (ICES) for providing data access.
Disclaimer: The opinions, results and conclusions reported in this paper are those of the authors and are independent from the funding sources. Parts of this material are based on data and information compiled and provided by the Canadian Institute of Health Information (CIHI). However, the analyses, conclusions, opinions, and statements expressed herein are those of the authors, and not necessarily those of CIHI. No endorsement by ICES, the Ontario Ministry of Health and Long-Term Care, or CIHI is intended or should be inferred.
Correspondence: Deshayne Fell, Children’s Hospital of Eastern Ontario Research Institute, 401 Smyth Road, Centre for Practice Changing Research, Room L-1154, Ottawa, Ontario, K1H 8L1, Canada. E-mail: email@example.com
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