Pregnant, Influenza-Infected, and Hospitalized
By Stan Deresinski, MD, FACP, FIDSA
Clinical Professor of Medicine, Stanford University
SYNOPSIS: Almost one-third of women age 15-44 years hospitalized with influenza were pregnant and almost 5% required intensive care.
SOURCE: Holstein R, Dawood FS, O’Halloran A, et al. Characteristics and outcomes of hospitalized pregnant women with influenza, 2010 to 2019: A repeated cross-sectional study. Ann Intern Med 2022;175:149-158.
Holstein et al evaluated pregnant women hospitalized with influenza. Cases were identified in FluSurv-NET, a network sponsored by the CDC conducting population-based surveillance of laboratory-confirmed influenza that encompasses approximately 9% of the U.S. population.1 The analysis included all women age 15-44 years with influenza during the 2010-2011 through 2018-2019 seasons who were pregnant at the time of admission and who resided within the designated catchment areas.
During the study period, 9,652 women in this age group were hospitalized with influenza and 2,690 of these were pregnant, although this proportion varied from 24.4% to 36% over the influenza seasons. A total of 62.3% were in the third trimester, and the median gestational age was 32 weeks. At least one underlying condition was present in 41.9%, including 22.8% with asthma, 13.7% with obesity, and 7.6% with chronic metabolic disease. In addition, 12.2% were current smokers.
In the total cohort, 846 had received an influenza vaccine at least 14 days before illness onset, but this proportion increased over the duration of the study, rising from 23.5% to 41.7%. The likelihood of vaccination correlated with age grouping: 29% in those age 15-34 years and 45.1% in the cohort of those age 35-44 years. Vaccination rates were highest in non-Hispanic white women (37.9%) and lowest in non-Hispanic Black women (19.3%).
The proportion of patients given antiviral therapy increased from 78.3% to 94.6% from 2010-2011 to 2019-2020. Overall, 88.1% received such treatment (oseltamivir in 99.8% of cases). Among the total group, 6.1% contracted pneumonia, 4.8% underwent intensive care, 1.8% received mechanical ventilation, and 0.3% died.
Influenza A and B accounted for 86.2% and 13.6% of infections, respectively. Among the former, 61.7% were influenza A H3N2 and 38.3% were influenza A H1N1. After adjustments, including for vaccination status, the risk ratio for severe outcomes for influenza A H1N1 infection vs. influenza A H3N2 was 1.9 (95% CI, 1.3- 2.8).
The median duration of hospitalization was two days and 28.0% were no longer pregnant at the time of discharge, most often (96.2%) because of live birth (although one newborn subsequently died while still in the hospital). Twenty-three pregnancies ended with fetal loss, with almost all losses occurring in the first two trimesters.
This study adds to our understanding of the effect on pregnant women of influenza virus infection severe enough to lead to hospitalization and illustrates several important points. Almost one-third of women of reproductive age hospitalized with influenza are pregnant, with approximately two-thirds of these in their third trimester. Two-fifths presented with at least one underlying condition, most often asthma, obesity, and/or chronic metabolic disease. Almost 5% required intensive care, with some requiring mechanical ventilation — 0.3% died, while 3% experienced fetal loss.
In this study, infection with influenza A H1N1 resulted in a greater risk of severe outcome than infection with influenza A H3N2. During the pandemic caused by influenza A(H1N1)pdm09, while pregnant women comprised only 1% of those infected in the United States, they accounted for 5% of influenza-related deaths.2 The first influenza season examined in this study was 2010-2011, a year dominated by the pandemic influenza A H1N1. By the end of the study period, the proportion of H1N1 had decreased to less than one-third. So far in 2021-2022, H1N1 infections are uncommon. It is estimated that, at any one time, 5% of women of reproductive age in the United States are pregnant. The potential for adverse outcomes in these patients makes the need for improved prevention obvious, and the best current way to do this is by vaccination. Therefore, improving the rate of vaccination in women of reproductive age is critical — as it is in the general population. However, in this regard the protective efficacy of influenza vaccination requires improvement. Better anti-influenza therapy, including ones approved for use in pregnancy, also is a critical need.3
- Centers for Disease Control and Prevention. Influenza Hospitalization Surveillance Network (FluSurv-NET).
- Siston AM, Rasmussen SA, Honein MA, et al; Pandemic H1N1 Influenza in Pregnancy Working Group. Pandemic 2009 influenza A(H1N1) virus illness among pregnant women in the United States. JAMA 2010;303:1517-1525.
- Chow EJ, Beigi RH, Riley LE, Uyeki TM. Clinical effectiveness and safety of antivirals for influenza in pregnancy. Open Forum Infect Dis 2021;8:ofab138.
Almost one-third of women age 15-44 years hospitalized with influenza were pregnant and almost 5% required intensive care.
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