Effects of influenza on pregnant women and infants Sonja A. Rasmussen, MD, MS; Denise J. Jamieson, MD, MPH; Timothy M. Uyeki, MD, MPH, MPP I nformed decisions regarding the use of influenza vaccine during preg- nancy must include consideration of both the benefits and the potential risks of the vaccine. To fully understand the benefits of influenza vaccine during pregnancy, we must examine the com- plications that are associated with influ- enza virus infection during pregnancy that include those in the woman and her infant. In addition, because observa- tional studies and 1 randomized trial have shown that influenza vaccination protects infants 6 months old from in- fluenza, 1-3 the influenza-associated dis- ease burden among young infants also must be considered. Here, we review data on the effects of influenza virus in- fection on pregnant women and infants 6 months old. Effects of influenza during pregnancy on the pregnant woman Pregnant women are more likely to ex- perience severe complications that are associated with influenza compared with the general population, based on data from seasonal influenza epidemics and pandemics. 4 During pregnancy, immuno- logic alterations and physiologic changes that affect respiratory, cardiovascular, and other organ systems place women at in- creased risk for certain infections and asso- ciated complications. A woman’s immune system adapts during pregnancy to tolerate a genetically foreign fetus. How this adap- tation occurs is not well understood, but it appears that a shift away from cell-medi- ated immunity and toward humoral im- munity occurs. This immunologic adapta- tion results in increasing the risk for complications that are associated with cer- tain infections, which includes infection with influenza viruses. 5 Changes in the cardiovascular and respiratory systems that include increased heart rate, stroke volume, oxygen consumption, and de- creased lung capacity also put women at increased risk for severe influenza illness. 6,7 Data suggest that pregnant women are at increased risk of hospitalization re- lated to complications from seasonal in- fluenza compared with nonpregnant women. Using data from the Tennessee Medicaid program for the years 1974- 1993, Neuzil et al 8 showed that pregnant women were more likely to be hospital- ized with an acute cardiopulmonary ill- ness during seasonal influenza epidem- ics compared with postpartum women. The highest rate of hospitalization was during the third trimester of pregnancy, at which time pregnant women were 3-4 times more likely to be hospitalized with a cardiopulmonary illness during influ- enza season compared with postpartum women. Cox et al 9 examined data from a na- tionally representative hospital dis- charge database for the years 1998-2002 and showed that the proportion of hos- pitalizations among pregnant women with respiratory illness was substantially higher during influenza season (3.4 per 1000 pregnancy hospitalizations, com- pared with 1.8 per 1000 during the rest of the year). Pregnant women with comor- bid conditions (ie, chronic cardiac dis- ease, chronic pulmonary disease, diabe- tes mellitus, chronic renal disease, malignancies, and immunosuppressive disorders) were 3 times more likely to be hospitalized for respiratory illness during influenza season than women without these comorbid conditions. A study from Nova Scotia also demon- strated an increased risk for influenza- associated complications during preg- nancy. 10 Compared with the year before pregnancy, pregnant women without comorbid conditions were 1.7 (95% confidence interval [CI], 1.0 –2.8), 2.1 (95% CI, 1.3–3.3), and 5.1 (95% CI, 3.6 – 7.3) times more likely to be hospitalized for respiratory illness during the influ- enza season during the first, second, and third trimesters, respectively. Pregnant women with 1 comorbid conditions (defined as preexisting diabetes mellitus, From the Influenza Coordination Unit, Office of Infectious Diseases (Dr Rasmussen), the Division of Reproductive Health, National Center for Chronic Disease Prevention and Health Promotion (Dr Jamieson) and the Influenza Division, National Center for Immunization and Respiratory Diseases (Dr Uyeki), Centers for Disease Control and Prevention, Atlanta, GA. Received March 18, 2012; revised May 8, 2012; accepted June 28, 2012. The findings and conclusions in this report are those of the authors and do not necessarily represent the official position of the National Vaccine Program Office, the Centers for Disease Control and Prevention, or the Food and Drug Administration. The authors report no conflict of interest. Reprints not available from the authors. 0002-9378/$36.00 Published by Mosby, Inc. http://dx.doi.org/10.1016/j.ajog.2012.06.068 Influenza vaccination during pregnancy has been shown to decrease the risk of influenza and its complications among pregnant women and their infants up to 6 months old. To adequately assess the benefits and potential risks that are associated with the use of influenza vaccine during pregnancy, it is necessary to examine the influenza-associated complications that occur among pregnant women and their children. Pregnant women have been shown to be at increased risk for morbidity and death with influenza illness during seasonal epidemics and pandemics. Newborn infants born to mothers with influ- enza during pregnancy, especially mothers with severe illness, are at increased risk of adverse outcomes, such as preterm birth and low birthweight. Infants 6 months old who experience influenza virus infection have the highest rates of hospitalization and death of all children. Here we review the risks for influenza-associated complications among preg- nant women and infants 6 months old. Key words: infant, influenza, pandemic, pregnancy, seasonal www. AJOG.org Supplement to SEPTEMBER 2012 American Journal of Obstetrics & Gynecology S3
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