Neonatal outcomes after antenatal influenza immunization during the 2009 H1N1 influenza pandemic: Impact on preterm birth, birth weight, and small for gestational age birth

Jennifer L. Richards, Craig Hansen, Christine Bredfeldt, Robert A. Bednarczyk, Mark C. Steinhoff, Dzifa Adjaye-Gbewonyo, Kevin Ault, Mia Gallagher, Walter Orenstein, Robert L. Davis, Saad B. Omer

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105 Citations (Scopus)


Background. Influenza infection during pregnancy is associated with adverse fetal outcomes such as preterm birth and small for gestational age (SGA). Maternal influenza immunization may prevent these adverse infant outcomes during periods of influenza circulation.Methods. We conducted a retrospective cohort study of live births within Kaiser Permanente (KP) Georgia and Mid-Atlantic States (n = 3327) during the period of 2009 influenza A (H1N1) virus circulation. Primary outcomes were third-trimester preterm birth (27-36 weeks), birth weight, low birth weight (LBW, <2500 g), and SGA.Results. There were 327 (9.8%) preterm, 236 (7.4%) LBW, and 267 (8.4%) SGA births. Among H1N1-vaccinated mothers (n = 1125), there were 86 (7.6%) preterm, 68 (6.4%) LBW, and 99 (9.3%) SGA births, and the mean birth weight was 3308.5 g (95% confidence interval [CI], 3276.6-3340.4). Among unvaccinated mothers (n = 1581), there were 191 (12.1%) preterm, 132 (8.8%) LBW, and 123 (8.2%) SGA births, and the mean birth weight was 3245.3 g (95% CI, 3216.5-3274.2). Infants of H1N1-vaccinated mothers had 37% lower odds of being born preterm than infants of unvaccinated mothers (adjusted odds ratio, 0.63 [95% CI,. 47-.84]). The mean birth weight difference between infants of H1N1-vaccinated mothers and infants of unvaccinated mothers was 45.1 g (95% CI, 1.8-88.3). There was no significant association between maternal H1N1 influenza immunization and LBW or SGA.Conclusions. Pregnant women who received H1N1 influenza vaccine were less likely to give birth preterm, and gave birth to heavier infants. The findings support US vaccine policy choices to prioritize pregnant women during the 2009 influenza A (H1N1) pandemic.

Original languageEnglish
Pages (from-to)1216-1222
Number of pages7
JournalClinical Infectious Diseases
Issue number9
Publication statusPublished or Issued - 1 May 2013
Externally publishedYes


  • infants
  • influenza vaccine
  • maternal immunization
  • pandemic influenza
  • prematurity

ASJC Scopus subject areas

  • Microbiology (medical)
  • Infectious Diseases

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