Researchers have long known that during the 1918 pandemic an extraordinarily high number of pregnant women died from the influenza, and those that survived endured a very high miscarriage rate. Again - during the much milder 1957 Asian Flu - pregnant women reportedly suffered disproportionately higher mortality rates than non-pregnant women of the same age.
In a Perspective, written by 3 CDC physicians (Sonja A. Rasmussen, Denise J. Jamieson, Joseph S. Bresee) and published in a 2008 CDC EID Journal article - Pandemic Influenza and Pregnant Women - we get this assessment of the historic impact of influenza on pregnant women.
Although appropriate nonpregnant control groups were generally not available, mortality rates among pregnant women in the pandemics of 1918 and 1957 appeared to be abnormally high (5,7). Among 1,350 reported cases of influenza among pregnant women during the pandemic of 1918, the proportion of deaths was reported to be 27% (5).More than a year later - during the 2009 H1N1 pandemic - we were reminded of the heavy toll that influenza infection can have on pregnant women, and their offspring.
Similarly, among a small case series of 86 pregnant women hospitalized in Chicago for influenza in 1918, 45% died (6). Among pregnancy-associated deaths in Minnesota during the 1957 pandemic, influenza was the leading cause of death, accounting for nearly 20% of deaths associated with pregnancy during the pandemic period; half of women of reproductive age who died were pregnant
- In the fall of 2009 we learned that pregnant women were six times more likely to be hospitalized than non-pregnant women (see Pregnancy & Flu: A Bad Combination).
- And in 2011, in BMJ: Perinatal Outcomes After Maternal 2009/H1N1 Infection we saw a study where pregnant women who were admitted to the hospital with an H1N1 infection experienced a 3 to 4 times higher rate of preterm birth, 4 to 5 times greater risk of stillbirth, and a 4 to 6 times higher rate of neonatal death.
Clinical Infectious Diseases: Flu Vaccine May Reduce Incidence of Stillbirth
JID: Benefits Of Early Use of Influenza Antivirals In Pregnancy
Despite ample evidence of harm to expectant mothers and their offspring from influenza infection - and studies that show the benefits of influenza vaccination to this vulnerable cohort - uptake of the flu vaccine among pregnant women is less than doctors would like to see.
Some of this reluctance is due to the persistent anti-vaccine rhetoric on the internet and in the media - including unfounded claims that the 2009 H1N1 vaccine was responsible for `thousands’ of miscarriages. The science, however, overwhelmingly supports the safety of flu shots for pregnant women.
- In October of 2011, in IDSA: Flu Vaccines In Pregnancy, we saw several studies presented at the 49th Annual Meeting of the Infectious Diseases Society of America (IDSA) reaffirming the benefits and safety of maternal vaccination.The CDC summarized those studies in Pregnancy and Influenza Vaccine Safety.
- In 2012, in BMJ: H1N1 Vaccination & Fetal Death Rates, we looked at a large cohort study of pregnant women in Denmark that reassuringly finds no increase in miscarriage among those who received the 2009 H1N1 vaccine. Results that mirrored another study we saw in Hong Kong: No Increase In Fetal Death Among Vaccine Recipients.
- And the CDC and ACIP continue to recommend the flu shot (see Pregnant Women & Influenza (Flu)) during any trimester of their pregnancy, citing a long safety record of the vaccine in pregnant women.
RESEARCH ARTICLEKim Newsome, C. J. Alverson, Jennifer Williams, Anne F. McIntyre, Anne D. Fine, Cathy Wasserman, Kathryn H. Lofy, Meileen Acosta, Janice K. Louie… See all authors
First published: 09 January 2019
Pregnant women with influenza are more likely to have complications, but information on infant outcomes is limited. Methods
Five state/local health departments collected data on outcomes of infants born to pregnant women with 2009 H1N1 influenza reported to the Centers for Disease Control and Prevention from April to December 2009. Collaborating sites linked information on pregnant women with confirmed 2009 H1N1 influenza, many who were severely ill, to their infants' birth certificates. Collaborators also collected birth certificate data from two comparison groups that were matched with H1N1‐affected pregnancies on month of conception, sex, and county of residence.
490 pregnant women with influenza, 1,451 women without reported influenza with pregnancies in the same year, and 1,446 pregnant women without reported influenza with prior year pregnancies were included. Women with 2009 H1N1 influenza admitted to an intensive care unit (ICU; n = 64) were more likely to deliver preterm infants (< 37 weeks), low birth weight infants, and infants with Apgar scores <= 6 at 5 min than women in comparison groups (adjusted relative risk, aRR = 3.9 [2.7, 5.6], aRR = 4.6 [2.9, 7.5], and aRR = 8.7 [3.6, 21.2], for same year comparisons, respectively). Women with influenza who were not hospitalized and hospitalized women not admitted to the ICU did not have significantly elevated risks for adverse infant outcomes.
Severely ill women with 2009 H1N1 influenza during pregnancy were more likely to have adverse birth outcomes than women without influenza, providing more support for influenza vaccination during pregnancy.
Newborns face risks when born to women with the flu
Pregnant women with influenza are more likely to experience complications, but how this affects infants is unclear. A new Birth Defects Research study uncovers the potential risks to infants.
The study included 490 pregnant women with influenza, 1451 women without influenza with pregnancies in the same year, and 1446 pregnant women without influenza with prior year pregnancies. Women with 2009 H1N1 influenza admitted to an intensive care unit were more likely to deliver preterm infants, low birth weight infants, and infants with low Apgar scores than women in the other groups.
Women with influenza who were not hospitalized, as well as hospitalized women not admitted to the intensive care unit, did not have significantly elevated risks for adverse infant outcomes.
"The message of this work is particularly timely in the midst of the current influenza season. Our study found that severely ill women with 2009 H1N1 influenza during pregnancy were more likely to have adverse birth outcomes--such as their baby being born preterm or of low birth weight--than women without influenza," said senior author Dr. Sonja Rasmussen, of the University of Florida. "These findings support the importance of pregnant women receiving the influenza vaccine and of prompt treatment with antiviral medications for pregnant women suspected of having influenza."
While the focus of influenza during pregnancy studies over the past decade have been primarily on the impact of maternal H1N1 infection, the 1957 H2N2 pandemic produced similar results, and so it can be assumed that other influenza subtypes may be able to produce similar - or possibly even worse - outcomes.