Infant Respiratory Outcomes Associated with Prenatal Exposure to Maternal 2009 A/H1N1 Influenza Vaccination

PLoS One. 2016 Aug 3;11(8):e0160342. doi: 10.1371/journal.pone.0160342. eCollection 2016.


Background: Infants are at high risk for influenza illness, but are ineligible for vaccination before 6 months. Transfer of maternal antibodies to the fetus has been demonstrated for 2009 A/H1N1 pandemic vaccines; however, clinical effectiveness is unknown. Our objective was to evaluate the association between 2009 A/H1N1 pandemic vaccination during pregnancy and rates of infant influenza and pneumonia.

Methods: We linked a population-based birth cohort to administrative databases to measure rates of influenza and pneumonia diagnosed during ambulatory physician visits, hospitalizations and emergency department visits during one year of follow-up. We estimated incidence rate ratios and 95% confidence intervals (95% CI) using Poisson regression, comparing infants born to A/H1N1-vaccinated women (vaccine-exposed infants) with unexposed infants, adjusted for confounding using high-dimensional propensity scores.

Results: Among 117,335 infants in the study, 36,033 (31%) were born to A/H1N1-vaccinated women. Crude rates of influenza during the pandemic (per 100,000 infant-days) for vaccine-exposed and unexposed infants were similar (2.19, 95% CI: 1.27-3.76 and 3.60, 95% CI: 2.51-5.14, respectively), as were crude rates of influenza and pneumonia combined. We did not observe any significant differences in rates of study outcomes between study groups during the second wave of the 2009 A/H1N1 pandemic, nor during any post-pandemic time period.

Conclusion: We observed no difference in rates of study outcomes among infants born to A/H1N1-vaccinated mothers relative to unexposed infants born during the second A/H1N1 pandemic wave; however, due to late availability of the pandemic vaccine, the available follow-up time during the pandemic time period was very limited.

MeSH terms

  • Adult
  • Cohort Studies
  • Female
  • Humans
  • Infant, Newborn
  • Infant, Newborn, Diseases / epidemiology
  • Infant, Newborn, Diseases / etiology
  • Influenza A Virus, H1N1 Subtype / immunology*
  • Influenza Vaccines / adverse effects
  • Influenza Vaccines / therapeutic use*
  • Influenza, Human / congenital
  • Influenza, Human / epidemiology*
  • Influenza, Human / prevention & control*
  • Male
  • Middle Aged
  • Pneumonia / congenital
  • Pneumonia / epidemiology*
  • Pneumonia / etiology
  • Pregnancy
  • Prenatal Exposure Delayed Effects / epidemiology*
  • Prenatal Exposure Delayed Effects / etiology
  • Prenatal Exposure Delayed Effects / immunology
  • Respiratory Distress Syndrome, Newborn / epidemiology
  • Respiratory Distress Syndrome, Newborn / etiology
  • Retrospective Studies
  • Treatment Outcome
  • Vaccination / adverse effects*
  • Vaccination / statistics & numerical data
  • Young Adult


  • Influenza Vaccines

Grant support

This work was supported by a Canadian Institutes of Health Research (CIHR) Grant # MOP-119601. CIHR had no role in conducting the study or in the decision to submit the article for publication.