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 Māori 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.