Monovalent H1N1 influenza vaccine safety in pregnant women, risks for acute adverse events

Objective: To assess risks for acute adverse events and pregnancy complications in pregnant women following monovalent 2009 H1N1 inactivated influenza (MIV) vaccination.

Methods: Within the Vaccine Safety Datalink, we compared rates of pre-specified medically attended events (MAE) occurring within 42 days of MIV vaccination to those occurring in matched cohorts that at the same gestational age were either unvaccinated or received seasonal trivalent inactivated influenza (TIV) vaccine. Using generalized estimating equation method, with a Poisson distribution and log link, we calculated adjusted incident rate ratios (AIRR).

Results: Among 9349 women receiving MIV in any trimester, only one MAE occurred 0-3 days following MIV, an allergic reaction. No cases of Guillain-Barré syndrome, Bell's palsy, or transverse myelitis occurred 1-42 days after MIV. Compared to women receiving TIV and to unvaccinated women, risks for acute MAEs were not increased following MIV for any outcome. Hyperemesis was the most common adverse event in the MIV, TIV, and unvaccinated groups, occurring at a rate of about 4% over a 42-day period in all groups. Over a 42-day window, among all groups, incident gestational diabetes occurred at a rate of 3% and thrombocytopenia occurred at a rate of approximately 0.3%. Among women receiving MIV during pregnancy, increased risks for these and other less common obstetric events were not detected.

Conclusion: In this large cohort of pregnant women no acute safety signals were identified within 6 weeks of receipt of MIV.