2009 Pandemic Influenza A (H1N1) Virus Infection Among Pregnant Women in the United States, April–August, 2009

Background: During previous influenza pandemics, mortality rates among pregnant women were elevated, and seasonal influenza data showed that pregnant women were at increased risk for hospitalization. Early 2009 pandemic influenza A (H1N1) [2009 H1N1] data suggest pregnant women are at increased risk of hospitalization and death, and experience delays in antiviral treatment.

Objectives: To describe 2009 H1N1 illness severity and the impact of early treatment with antiviral medications among pregnant women in the United States.

Methods: CDC requested surveillance data from all state health departments and the local health departments for Chicago, New York City and the District of Columbia on all pregnant women reported with confirmed or probable 2009 H1N1 from April to August 2009. We compared illness severity outcomes among those treated with antiviral medications early (≤2 days from symptom onset), intermediate (3-4 days from symptom onset) and late (>4 days from symptom onset).

Results: We received reports on 774 pregnant women in the United States with 2009 H1N1 illness, including 28 deaths, which represented 6% of all 2009 H1N1 reported deaths in this period. Among 496 hospitalized pregnant cases, 103 (21%) were admitted to intensive care.  Pregnant women with late antiviral treatment were more likely to be admitted to intensive care (relative risk [RR]=6.7, 95% confidence interval [CI] 3.6-12.3) and mechanically ventilated (RR=14.1, 95% CI 5.8-34.4) than those treated early.  These risk estimates decreased by half among pregnant women treated intermediately. Among the deaths, none received antiviral treatment within 2 days of symptom onset.

Conclusions: Pregnant women continue to be disproportionately represented among those with severe illness and death due to 2009 H1N1. This study provides strong evidence to support prompt empiric antiviral treatment for pregnant women with manifestations of influenza and vaccination with 2009 H1N1 influenza vaccine and seasonal influenza vaccine.