Impact of influenza circulation in the community on the risk of maternal morbidity and preterm birth

The Advisory Committee on Immunization Practices recommends that influenza vaccination be administered to healthy pregnant women who expect to be past their 14th gestational week during influenza season. To assess the potential benefit of this policy, we conducted a retrospective cohort study of pregnant women that estimated the risk of influenza-related medical encounters and preterm delivery comparing periods with and without influenza isolate activity in the community. This study was based on isolate surveillance data between October 1–May 31 and diagnostic data for healthy women enrolled in Group Health Cooperative between June 1, 1991–December 31, 1997.; We used repeated measures logistic regression models to estimate the log odds of an influenza-like illness (ILI) episode on a week-by-week basis for the combined set of pregnancy weeks for each woman as a function of isolate activity among women in the same pregnancy stage. We detected increasingly large adjusted odds ratios (ORs) for ILI episodes as pregnancy stage progressed for periods of heightened influenza isolate detection. We also determined that approximately 99% of all diagnoses occurred during outpatient visits. We identified the following ORs (95% CI) for ILI episodes during periods of high influenza intensity compared to no activity: 1.88 (1.25–2.83), 1.71 (1.22–2.40), 2.34 (1.60–3.42), and 2.99 (1.80–4.98) during the 1st–3rd trimesters and postpartum period.; We assessed the relationship between isolate activity and preterm birth by running predictive logistic regression models for a variety of gestational time windows. We identified modest associations that were not consistently elevated. We detected moderate adverse associations between non-influenza isolates during gestational weeks 27–31 and preterm birth between 32–36 weeks (OR = 1.40, 1.02–1.93). We also detected associations between influenza within 2 weeks of delivery, as well as during weeks 11–14, and medically indicated preterm birth (OR = 1.96, 1.07–3.60 and OR = 2.87, 1.47–5.60).; Our findings support current influenza vaccine recommendations for pregnant women while identifying novel associations between isolate activity and postpartum risk of an ILI episode, as well as a risk among medically indicated preterm birth. Further research concerning the effect of respiratory infections among high-risk pregnancies may provide valuable information on the prevention of early delivery.