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BACKGROUND Pregnant women are considered to have a high risk for influenza virus infection, although little is known about the biological reasons for this risk. Antiviral immunity is critical during influenza virus infection, and understanding the changes that occur during pregnancy and the effect of vaccination is essential for improving health outcomes for mother and baby. METHODS Peripheral blood mononuclear cells (PBMCs) were isolated from 26 healthy, nonpregnant women and 28 healthy pregnant women and cultured with 2009 pandemic influenza A virus subtype H1N1 (H1N1/09). Protein concentrations of interferon α (IFN-α), IFN-λ, and IFN-γ were measured from culture supernatant. Messenger RNA expression of protein kinase R (PKR) and Toll-like receptors 3, 7, and 9 was also measured from cell lysates. RESULTS PBMCs from pregnant women produced significantly less IFN-α (median level, 114.06 pg/mL [range, 51.48-394.9]) and IFN-λ (median level, 30.65 pg/mL [range, 0-260]), compared with PBMCs from nonpregnant women (median level, 800.38 pg/mL [range, 259-1458] and 479.87 pg/mL [257.1-1113], respectively; P < .01). PKR expression was also significantly reduced in PBMCs from pregnant women (P < .05). Vaccination significantly improved innate and adaptive immunity in pregnancy (P < .01). CONCLUSION PBMCs from nonvaccinated pregnant women have attenuated antiviral immunity following H1N1/09 stimulation, but vaccination improves this response. These novel findings help improve understanding of the increased susceptibility and disease severity to influenza virus infection during pregnancy and the importance of influenza vaccination.