Immunisation against influenza during pregnancy

British Medical Journal
19 May 2012 (Vol 344, Issue 7857)

Immunisation against influenza during pregnancy
BMJ 2012; 344 doi: 10.1136/bmj.e3091 (Published 2 May 2012)
Marian Knight, NIHR (National Institute for Health Research) research professor in public health 1, Boon Lim, consultant obstetrician2

The benefits outweigh the risks
Since the A/H1N1 2009 influenza pandemic, universal immunisation of pregnant women against seasonal flu has been recommended in many areas of the world.1 2 3 Despite experience with immunisation against seasonal flu in pregnancy over many years, uptake of influenza vaccine in pregnancy during the 2009 A/H1N1 2009 pandemic was low and immunisation rates among pregnant women generally remain low.4 5 One commonly cited reason for this is concern among women and clinical staff about the safety of the vaccine during pregnancy.6

In a linked research paper (doi:10.1136/bmj.e2794), Pasternak and colleagues present findings from an important new Danish national cohort study of women vaccinated against influenza A/H1N1 2009.7 The study suggests that women who are immunised in pregnancy have a lower risk of fetal loss than non-immunised women. The study provides reassuring information for people who are worried about the safety of the vaccine and evidence of the benefits of vaccination, which were previously only hypothesised. This is particularly important because the influenza vaccination season has just started in the southern hemisphere, where the A/California/7/2009 strain is included in the current vaccine. After a World …

Vaccination against pandemic A/H1N1 2009 influenza in pregnancy and risk of fetal death: cohort study in Denmark
BMJ 2012; 344 doi: 10.1136/bmj.e2794 (Published 2 May 2012)
Björn Pasternak, registrar and postdoctoral fellow12, Henrik Svanström, statistician1, Ditte Mølgaard-Nielsen, researcher1, Tyra G Krause, consultant3, Hanne-Dorthe Emborg, epidemiologist3, Mads Melbye, professor1, Anders Hviid, senior investigator1

Abstract  [Open Access]
Objective  To investigate whether an adjuvanted pandemic A/H1N1 2009 influenza vaccine in pregnancy was associated with an increased risk of fetal death.

Design Nationwide register based cohort study.

Setting  Denmark.

Participants All clinically recognised singleton pregnancies that ended between November 2009 and September 2010. Individual level data on exposure to an inactivated AS03 pandemic A/H1N1 2009 influenza vaccine (Pandemrix) and potential confounders were linked to the study cohort using a unique person identifier.

Main outcome measures The primary outcome measure was risk of fetal death (spontaneous abortion and stillbirth combined) in H1N1 vaccinated compared with unvaccinated pregnancies, adjusting for propensity scores. Secondary outcome measures were spontaneous abortion (between seven and 22 weeks’ gestation) and stillbirth (after 22 completed weeks’ gestation).

Results  The cohort comprised 54 585 pregnancies; 7062 (12.9%) women were vaccinated against pandemic A/H1N1 2009 influenza during pregnancy. Overall, 1818 fetal deaths occurred (1678 spontaneous abortions and 140 stillbirths). Exposure to the H1N1 vaccine was not associated with an increased risk of fetal death (adjusted hazard ratio 0.79, 95% confidence interval 0.53 to 1.16), or the secondary outcomes of spontaneous abortion (1.11, 0.71 to 1.73) and stillbirth (0.44, 0.20 to 0.94). Estimates for fetal death were similar in pregnant women with (0.82, 0.44 to 1.53) and without comorbidities (0.77, 0.47 to 1.25).

Conclusion  This large cohort study found no evidence of an increased risk of fetal death associated with exposure to an adjuvanted pandemic A/H1N1 2009 influenza vaccine during pregnancy.