Hospitalized Pregnant Women With H1N1: Poorer Outcomes Than H3N2

NEW YORK (Reuters Health) – Hospitalized pregnant women with the H1N1 flu strain are more likely to experience severe adverse outcomes than those with H3N2, a new study suggests.

An analysis of data from 9,652 women hospitalized with influenza revealed that pregnant women with influenza A H1N1 had a higher risk of being admitted to the intensive-care unit (ICU) and dying, researchers reported in the Annals of Internal Medicine.

They also note that a third of hospitalized reproductive-aged women with influenza were pregnant, which seems very high given that, on average, just 5% of women in this age group are pregnant at any time in the U.S.

“Pregnant women infected with influenza A(H1N1) had almost twice the risk of severe maternal in-hospital outcomes compared with pregnant women infected with influenza A(H3N2),” said the study’s first author, Dr. Rachel Holstein of the Centers for Disease Control and Prevention, in Atlanta.

“Only 32% of pregnant people hospitalized with flu in this study had received a flu vaccine,” she told Reuters Health by email. “Although this study was not designed to assess vaccine effectiveness, in adjusted analyses, pregnant women hospitalized with flu who were vaccinated had a significantly lower risk of severe flu outcomes compared with those who were not vaccinated. Pregnant women are a high-priority group for flu vaccination, and these data highlight opportunities to improve influenza vaccine uptake among pregnant women.”

To take a closer look at the impact of the flu on pregnant women, researchers conducted a repeated cross-sectional analysis of data from FluSurv-NET, a large multicenter CDC sponsored network that conducts population-based surveillance for laboratory-confirmed influenza-associated hospitalizations in all age groups and represents about 9% of the U.S. population.

Of the women aged 15 to 44 who were hospitalized with influenza during the flu seasons running from 2010-2011 through 2018-2019, 2,690 were pregnant. Their median age was 28, with 36% non-Hispanic white, 29% non-Hispanic Black and 20% Hispanic or Latino.

Most of the women (62%) were in their third trimester and 42% had one or more underlying medical conditions, such as asthma, chronic metabolic disease and obesity. Just 32% were vaccinated and 88% received antiviral treatment.

Overall, 5% were admitted to the ICU, 2% required mechanical ventilation, and 0.3% died. After adjustment for influenza season, surveillance site, age, pregnancy trimester and vaccination status, those with H1N1 were more likely to have severe outcomes compared with those with H3N2 (adjusted risk ratio, 1.9; 95% CI, 1.3 to 2.8).

Most were still pregnant at discharge and 96% had a pregnancy resulting in a live birth, while 3% had fetal loss.

“Flu is especially dangerous for pregnant people because changes in the immune system, heart, and lungs during pregnancy (and up to two weeks after delivery) can make people more vulnerable to flu and its potentially severe complications,” Dr. Holstein said. “An annual flu vaccine is the best way to protect pregnant people and their babies from flu.”

The study should spur all healthcare providers to look for opportunities to vaccinate women who are pregnant or who could be pregnant, said Dr. Stuart Ray, a professor of medicine in the division of infectious diseases at Johns Hopkins Medicine, in Baltimore, Maryland.

“Healthcare providers should also make sure they initiate antiviral therapy promptly in women who have influenza during pregnancy,” Dr. Ray told Reuters Health by phone. “This study was done prior to COVID-19 and the question is how to approach people with an influenza-like illness in COVID times. There is a need to be vigilant for both infections.”

“The troubling message from the study is that there is a disparity in vaccination rates, with women who are Hispanic or nonwhite having lower rates,” said Dr. Ray, who was not involved in the study. “It’s a gap we should fill.”

It’s been known for some time that pregnant patients are more susceptible to the flu and have higher rates of hospitalization and serious morbidity compared with women of comparable ages who are not pregnant, said Dr. Frederick Friedman, an associate professor and vice chair of academic affairs for the department of obstetrics, gynecology and reproductive science at the Icahn School of Medicine at Mount Sinai and director of the ob-gyn service at Elmhurst Hospital Center, in New York City.

“The researchers found about a two-fold increase in the rate of severe outcomes with H1N1,” Dr. Friedman told Reuters Health by email. “In the past, studies have found higher increased risk – as high as seven-fold. So it’s highly recommended that pregnant women get vaccinated. Obviously, it doesn’t completely prevent infection but it does seem to decrease infection rates and severity.”

SOURCE: and Annals of Internal Medicine, online December 27, 2021.

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