Influenza-Associated Outcomes Among Pregnant, Postpartum, and Nonpregnant Women of Reproductive Age

Background. Pregnant women are prioritized for seasonal influenza vaccination, but the evidence on the risk of influenza during pregnancy that is used to inform these policies is limited. Methods. Individual-level administrative data sets and active surveillance data were joined to estimate influenza-associated hospitalization and outpatient visit rates by pregnancy, postpartum, and trimester status. Results. During 2012-2015, 46 of 260 (17.7%) influenza-confirmed hospitalizations for acute respiratory infection and 13 of 294 (4.4%) influenza-confirmed outpatient visits were among pregnant and postpartum women. Pregnant and postpartum women experienced higher rates of influenza-associated hospitalization than nonpregnant women overall (rate ratio [RR], 3.4; 95% confidence interval [CI], 2.5-4.7) and by trimester (first, 2.5 [95% CI, 1.2-5.4]; second, 3.9 [95% CI, 2.4-6.3]; and third, 4.8 [95% CI, 3.0-7.7]); the RR for the postpartum period was 0.7 (95% CI, 3.0-7.7). Influenza A viruses were associated with an increased risk (RR for 2009 pandemic influenza A[H1N1] virus, 5.3 [95% CI, 3.2-8.7]; RR for influenza A(H3N2) virus, 3.0 [95% CI, 1.8-5.0]), but influenza B virus was not (RR, 1.8; 95% CI,.7-4.6). Influenza-associated hospitalization rates in pregnancy were significantly higher for Maori women (RR, 3.2; 95% CI, 1.3-8.4), compared with women of European or other ethnicity. Similar risks for influenza-confirmed outpatient visits were not observed. Conclusion. Seasonal influenza poses higher risks of hospitalization among pregnant women in all trimesters, compared with nonpregnant women. Hospitalization rates vary by influenza virus type and ethnicity among pregnant women.