Health Outcomes After In-Utero Exposure to the 2009 Pandemic H1N1 Vaccine

pregnant woman receiving influenza vaccine
pregnant woman receiving influenza vaccine
There were no associations found between exposure to the 2009 pandemic H1N1 influenza vaccine during pregnancy and most 5-year pediatric outcomes.

There were no associations found between exposure to the 2009 pandemic H1N1 (pH1N1) influenza vaccine during pregnancy and most 5-year pediatric outcomes, though small associations were seen with reduced gastrointestinal infections and increased asthma, and further research is needed to confirm findings, according to a study published in The BMJ.

This retrospective cohort study was designed to identify any possible associations between exposure to the pH1N1 influenza vaccine during pregnancy and negative pediatric health outcomes. Data were taken from a population-based birth registry in Ontario, Canada, linked with health administrative databases. The majority of infants who’s mothers had documented vaccination for pH1NI, who were born between November 2009 and October 2010 and survived past the first day of life were included (N=104,249); investigators noted that approximately 13.5% of infants born during the study period were excluded as a result of administrative reasons. Of note, 31,295 women received the pH1N1 vaccine during pregnancy.

Members of the study population were followed until age 5 years, to determine rates of immune-related (asthma, infectious diseases) and nonimmune related outcomes (sensory disorders, neoplasms) as well as nonspecific morbidity outcomes (pediatric complex chronic conditions, inpatient or urgent health services use), mortality at younger than 5 years, and all-cause injuries. Hazard ratios, risk ratios, and incidence rate ratios were adjusted for potential confounding variables with propensity score weighting.

No significant associations were found between exposure and otitis media, upper or lower respiratory infections, any infectious diseases, sensory disorders, neoplasms, pediatric complex chronic conditions, inpatient and urgent health services use, or mortality at younger than 5 years. These findings remained unchanged in sensitivity analyses that accounted for potential confounders.

A weak, but statistically significant, association was observed between in-utero pH1N1 exposure and decreased rates of gastrointestinal infections (adjusted incidence rate ratio [IRR] 0.94; 95% CI, 0.91-0.98), and increased risk for asthma (adjusted hazard ratio [aHR] 1.05; 95% CI, 1.02-1.09). When a Bonferroni correction was used to account for multiplicity, neither of these associations remained statistically significant (adjusted IRR for gastrointestinal infections 0.94; 95% CI, 0.88-1.00; aHR for asthma 1.05; 95% CI, 1.00-1.11). A small, but statistically significant, increase was seen in the rate of all-cause injuries in exposed infants compared with unexposed infants, and this persisted in all sensitivity analyses (adjusted IRR 1.03; 95% CI, 1.01-1.05).

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Study investigators concluded, “Overall, our findings indicate that 2009 pH1N1 influenza vaccination in pregnancy was not associated with negative 5-year health outcomes in children, which is reassuring and consistent with a similar recent study from Denmark…Future studies in different settings and with different influenza vaccine formulations will be important for developing the evidence base on longer term pediatric outcomes following influenza vaccination during pregnancy.”

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Walsh LK, Donelle J, Dodds L, et al. Health outcomes of young children born to mothers who received 2009 pandemic H1N1 influenza vaccination during pregnancy: retrospective cohort study. BMJ. 2019;366:l4151.