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.