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. 2012 Dec 20;7(12):e51734.
doi: 10.1371/journal.pone.0051734. Epub 2012 Dec 10.

Evaluating the hazard of foetal death following H1N1 influenza vaccination; a population based cohort study in the UK GPRD

Cormac J Sammon  1 , Julia Snowball, Anita McGrogan, Corinne S de Vries
Affiliations

Evaluating the hazard of foetal death following H1N1 influenza vaccination; a population based cohort study in the UK GPRD

Cormac J Sammon et al. PLoS One. .

Abstract

Background: To evaluate the risk of foetal loss associated with pandemic influenza vaccination in pregnancy. Retrospective cohort study. UK General Practice Research Database Pregnancies ending in delivery or spontaneous foetal death after 21 October 2009 and starting before 01 January 2010.

Methodology/principal findings: Hazard ratios of foetal death for vaccinated compared to unvaccinated pregnancies were estimated for gestational weeks 9 to 12, 13 to 24 and 25 to 43 using discrete-time survival analysis. Separate models were specified to evaluate whether the potential effect of vaccination on foetal loss might be transient (for ~4 weeks post vaccination only) or more permanent (for the duration of the pregnancy). 39,863 pregnancies meeting our inclusion criteria contributed a total of 969,322 gestational weeks during the study period. 9,445 of the women were vaccinated before or during pregnancy. When the potential effect of vaccination was assumed to be transient, the hazard of foetal death during gestational weeks 9 through 12 (HR(unadj) 0.56; CI(95) 0.43 to 0.73) and 13 through 24 (HR(unadj) 0.45; CI(95) 0.28 to 0.73) was lower in the 4 weeks after vaccination than in other weeks. Where the more permanent exposure definition was specified, vaccinated pregnancies also had a lower hazard of foetal loss than unvaccinated pregnancies in gestational weeks 9 through 12 (HR(unadj) 0.74; CI(95) 0.62 to 0.88) and 13 through 24 (HR(unadj) 0.59; CI(95) 0.45 to 0.77). There was no difference in the hazard of foetal loss during weeks 25 to 43 in either model. Sensitivity analyses suggest the strong protective associations observed may be due in part to unmeasured confounding.

Conclusions/significance: Influenza vaccination during pregnancy does not appear to increase the risk of foetal death. This study therefore supports the continued recommendation of influenza vaccination of pregnant women.

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Conflict of interest statement

Competing Interests: The authors have read the journal's policy and have the following conflicts to declare: no support from any organisation for the submitted work; CdV has carried out work for GSK Pharmaceuticals in the past five years, no other authors have financial relationships with any organisations that might have an interest in the submitted work, no other relationships or activities that could appear to have influenced the submitted work. This does not alter the authors' adherence to all the PLOS ONE policies on sharing data and materials.

Figures

Figure 1
Figure 1. Time periods for inclusion of pregnancies in the study.
Figure 2
Figure 2. Percentage of pregnancies surviving (blue) and vaccinated (red) by each gestational week.
The drop in survival at 10 weeks is an artefact of the defaulting process. In reality the losses contributing to this curve would be more evenly distributed across weeks 9–12 resulting in a more gradual drop in survival.
See this image and copyright information in PMC

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Grants and funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors. CJS was funded by a University Research Studentship from the University of Bath. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.