title: Risk of fetal death after pandemic influenza virus infection or vaccination.
abstract: BACKGROUND
  During the 2009 influenza A (H1N1) pandemic, pregnant women were at risk for severe influenza illness. This concern was complicated by questions about vaccine safety in pregnant women that were raised by anecdotal reports of fetal deaths after vaccination.
  
  
  METHODS
  We explored the safety of influenza vaccination of pregnant women by linking Norwegian national registries and medical consultation data to determine influenza diagnosis, vaccination status, birth outcomes, and background information for pregnant women before, during, and after the pandemic. We used Cox regression models to estimate hazard ratios for fetal death, with the gestational day as the time metric and vaccination and pandemic exposure as time-dependent exposure variables.
  
  
  RESULTS
  There were 117,347 eligible pregnancies in Norway from 2009 through 2010. Fetal mortality was 4.9 deaths per 1000 births. During the pandemic, 54% of pregnant women in their second or third trimester were vaccinated. Vaccination during pregnancy substantially reduced the risk of an influenza diagnosis (adjusted hazard ratio, 0.30; 95% confidence interval [CI], 0.25 to 0.34). Among pregnant women with a clinical diagnosis of influenza, the risk of fetal death was increased (adjusted hazard ratio, 1.91; 95% CI, 1.07 to 3.41). The risk of fetal death was reduced with vaccination during pregnancy, although this reduction was not significant (adjusted hazard ratio, 0.88; 95% CI, 0.66 to 1.17).
  
  
  CONCLUSIONS
  Pandemic influenza virus infection in pregnancy was associated with an increased risk of fetal death. Vaccination during pregnancy reduced the risk of an influenza diagnosis. Vaccination itself was not associated with increased fetal mortality and may have reduced the risk of influenza-related fetal death during the pandemic. (Funded by the Norwegian Institute of Public Health.).
authors: Norwegian Institute of Public Health, Oslo, Norway. siri.haberg@fhi.no
authors: Siri E
authors: SE
authors: Håberg
authors: Lill
authors: L
authors: Trogstad
authors: Nina
authors: N
authors: Gunnes
authors: Allen J
authors: AJ
authors: Wilcox
authors: Håkon K
authors: HK
authors: Gjessing
authors: Sven Ove
authors: SO
authors: Samuelsen
authors: Anders
authors: A
authors: Skrondal
authors: Inger
authors: I
authors: Cappelen
authors: Anders
authors: A
authors: Engeland
authors: Preben
authors: P
authors: Aavitsland
authors: Steinar
authors: S
authors: Madsen
authors: Ingebjørg
authors: I
authors: Buajordet
authors: Kari
authors: K
authors: Furu
authors: Per
authors: P
authors: Nafstad
authors: Stein Emil
authors: SE
authors: Vollset
authors: Berit
authors: B
authors: Feiring
authors: Hanne
authors: H
authors: Nøkleby
authors: Per
authors: P
authors: Magnus
authors: Camilla
authors: C
authors: Stoltenberg
mesh_terms: D000293:Adolescent; D000328:Adult; D005260:Female; D005313:Fetal Death / Q000209:etiology / Q000517:prevention & control*; D006801:Humans; D053118:Influenza A Virus, H1N1 Subtype; D007252:Influenza Vaccines / Q000009:adverse effects; D007251:Influenza, Human / Q000150:complications* / Q000453:epidemiology / Q000517:prevention & control; D008875:Middle Aged; D009664:Norway / Q000453:epidemiology; D058873:Pandemics; D011247:Pregnancy; D011251:Pregnancy Complications, Infectious / Q000517:prevention & control; D016016:Proportional Hazards Models; D012306:Risk; D055815:Young Adult
chemical_list: D007252:Influenza Vaccines