A systematic review and meta-analysis of fetal outcomes following the administration of influenza A/H1N1 vaccination during pregnancy

Background: Pregnant women were identified as a population of priority for vaccination during the H1N1 influenza pandemic outbreak in 2009.

Objectives: To assess adverse fetal outcomes following the administration of H1N1 pandemic vaccination during pregnancy.

Search strategy: PubMed, Embase, and Cochrane Library were searched up to January 2017.

Selection criteria: Cohort studies investigating fetal outcomes after H1N1 influenza vaccination during pregnancy were eligible. The language was limited to English.

Data collection and analysis: Pairs of reviewers independently screened studies for eligibility, assessed the risk of bias, and extracted data from the included studies.

Main results: A total of 19 cohort studies were eligible. The use of vaccines during any period of pregnancy was associated with lower risk of stillbirth (adjusted hazard ratio 0.80, 95% confidence interval 0.69-0.92). No significant differences were found between the vaccinated versus unvaccinated groups in terms of the risks of spontaneous abortion, premature birth, and small for gestational age.

Conclusions: The administration of H1N1 vaccines during pregnancy might reduce the risk of stillbirth, a complication associated with H1N1 infection. The quality of evidence was, however, not adequate to reach a definitive conclusion.