Pregnancy and the Flu
Pregnant women, even those in perfect health, can become very ill from influenza (the "flu.")
For example, one large study demonstrated the detrimental effect of the flu on pregnant women over the course of 17 seasons (1). The study found that pregnant women in the third trimester during flu season were just as likely to be hospitalized for heart or lung problems as were women with serious, chronic medical conditions who were not pregnant. The risk increased substantially in late pregnancy. Indeed, healthy, pregnant women at 37 to 42 weeks gestation were 3-4 times more likely to be admitted to the hospital during the flu season for heart or lung problems than women who were 1-6 months postpartum. Pregnant women with asthma are at especially high risk for hospitalization from flu according to another study (2). In addition, the pandemic H1N1 flu virus of 2009 proved to be particularly harmful to pregnant women (3). Since the 2009-2010 pandemic, this H1N1 virus has continued to circulate during flu season (as a seasonal virus) and was predominant for most of the 2013-2014 flu season.
Because of the risk posed by flu to pregnant women and their infants, the Centers for Disease Control and Prevention (CDC) recommends that women who are or will be pregnant during the flu season get vaccinated against the flu (4). The American College of Obstetricians and Gynecologists (ACOG) strongly supports this recommendation (5).
Will the flu vaccine help?
Evidence continues to mount that the flu vaccine provides protection for both the pregnant woman and her child. While an earlier observational study (with methodological flaws) questioned the benefit of flu vaccine in pregnancy (6), a randomized controlled trial in Bangladesh showed that the flu vaccine protected both the mother and the infant from the flu when the mother was vaccinated (7). In addition, two newer studies support the benefit of flu vaccination during pregnancy. One study revealed a sharp reduction in flu season hospitalizations among infants whose mothers were vaccinated against the flu (8), while the other showed a decreased risk of premature and small-for-gestational-age births (9).
Is the flu vaccine safe for pregnant women?
Because the flu shot is an inactivated vaccine and contains no live virus, vaccine experts believe it is extremely safe for pregnant women. An early study of the flu shot in more than 2,000 pregnant women revealed no excess malignancies in the fetus (10). A similar but smaller study revealed no harmful effects of the flu vaccine on the fetus or the mother (11). More recent research found no serious adverse effects from the flu vaccine in the perinatal period or during the first six months of infant life (12). While the numbers of patients in these studies are relatively small, the results are reassuring. Subsequent studies with large numbers of pregnant women demonstrating the safety of flu vaccine are cited below.
Flu vaccine that is preservative-free will be offered to pregnant women at all PAMF locations. Pregnant women should not receive the live, intranasal flu vaccine. (12)
What do physicians think?
The vast majority of physician experts, including those at the CDC, the WHO, and the American College of Obstetricians and Gynecologists, believe the flu shot is safe and protective for pregnant women and their infants and can be given in any stage of pregnancy. PAMF strongly endorses this position. If you are pregnant and would like to discuss the flu vaccine further, please consult your primary care provider, obstetrician, or one of PAMF's flu vaccine experts.
Research on the 2009 H1N1 flu pandemic demonstrated the severe effects of this virus on pregnant and early postpartum women (13), while other studies demonstrated the H1N1 vaccine’s effectiveness in protecting mothers and infants against the H1N1 flu (14, 15), and the flu vaccine’s excellent safety record (16, 17). More recently, numerous cohort studies of thousands of pregnant women have demonstrated the safety of flu vaccine for the fetus (18, 19, 20, 21, 24) and the mother (20, 22, 23, 25), as well as a substantial reduction in influenza among vaccinated pregnant women (20, 26).
(1) Neuzil, K.M., Reed, G.W., Mitchel, E.F., Simonsen, L., Griffin , M.R. 1998. "Impact of influenza on acute cardiopulmonary hospitalizations in pregnant women." American Journal of Epidemiology, 148:1094-102.
(2) Hartert, T., Neuzil, K., Shintani, A., Mitchel, E., Snowden, M., Wood, L., Dittus, R., Griffin , M. 2003. "Maternal morbidity and perinatal outcomes among pregnant women with respiratory hospitalizations during influenza season." American Journal of Obstetric Gynecology, 189:1705-12.
(3) Jamieson, D.J., Honein, M.A., Rasmussen, S.A., et al. 2009. "H1N1 2009 influenza virus infection during pregnancy in the USA." Lancet , 374:451-458.
(4) Centers for Disease Control and Prevention. "Prevention and control of influenza with vaccines: recommendations of the Advisory Committee on Immunization Practices (ACIP), 2010." MMWR 2010; 59 (No. RR-8): 1-63.
(5) ACOG Committee on Obstetric Practice and Immunization Expert Work Group. 2014. Influenza vaccination and treatment during pregnancy. ACOG Committee Opinion No. 608, September 2014.
(6) Black, S., Shinefield, H., France, E., Fireman, B., Platt, S., Shay, D. 2004. Vaccine Safety Datalink Workgroup. "Effectiveness of influenza vaccine during pregnancy in preventing hospitalizations and outpatient visits for respiratory illness in pregnant women and their infants." American Journal of Perinatology, 21:333-9.
(7) Zaman K, Roy E, Arifeen S, Rahman M, Raqib R, Wilson E, et al. "Effectiveness of maternal influenza immunization in mothers and infants." N Engl J Med [ 10.1056/NEJMoa0708630 ]. 2008 Sept [cited 2008 October 1]. Available from the New England Journal of Medicine.
(8) Benowitz I, Esposito D, Gracey K, Shapiro E, Vazquez M. Influenza vaccine given to pregnant women reduces hospitalization due to influenza in their infants. Clin Infect Dis 2010; 51: 1355-1361.
(9) Omer SB, Goodman D, Steinhoff MC, Rochat R, Klugman KP, et al. (2011) Maternal Influenza Immunization and Reduced Likelihood of Prematurity and Small for Gestational Age Births: A Retrospective Cohort Study. PLoS Med 8(5): e1000441. doi:10.1371/journal.pmed.1000441
(10) Heinonen, O.P., Shapiro, S., Monson, R.R., Hartz , S.C. , Rosenberg , L., Slone, D. 1973. "Immunization during pregnancy against poliomyelitis and influenza in relation to childhood malignancy." International Journal of Epidemiology, 2:229-35.
(11) Deinard, A.S., Ogburn, P. 1981. A/NJ/8/76 influenza vaccination program: effects on maternal health and pregnancy outcome. American Journal of Obstetric Gynecology, 140:240-5.
(12) Munoz, F., Greisinger, A., Wehmanen, O., Mouzoon, M., Hoyle, J., Smith, F., Glezen, W. 2005. Safety of influenza vaccination during pregnancy. American Journal of Obstetric Gynecology, 192:1098-106.
(13) CDC. Maternal and infant outcomes among severely ill pregnant and postpartum women with 2009 pandemic influenza A (H1N1)—United States, April 2009-August 2010. MMWR 2011; 60:1193-6.
(14) Savelescu J et al. Using surveillance data to estimate pandemic vaccine effectiveness against laboratory confirmed influenza A (H1N1) 2009 infection: two case-control studies, Spain, season2009-2010. BMC Public Health 2011; 11:899.
(15) Fell D et al. H1N1 influenza vaccination during pregnancy and fetal and neonatal outcomes. Am J Public Health 2012; 102:e33-40.
(16) Moro P et al. Adverse events following administration to pregnant women of influenza A (H1N1) monovalent vaccine reported to the Vaccine Adverse Event Reporting System. Am J Obstet Gynecol 2011; 205:473.e1-9.
(17) Pasternak B et al. Vaccination against pandemic A/H1N1 influenza in pregnancy and risk of fetal death: a cohort study in Denmark. BMJ 2012; 344:e2794. doi: 10.1136/bmj.e2794.
(18) Sheffield J et al. Effect of influenza vaccination in the first trimester of pregnancy. Obstet Gynecol 2012;120:532-7.
(19) Irving S et al. Trivalent inactivated influenza vaccine and spontaneous abortion. Obstet Gynecol 2013;121:159-65.
(20) Haberg S et al. Risk of fetal death after pandemic influenzavirus infection or vaccination. New Engl J Med 2013;368: 333-40.
(21) Richards J et al. Neonatal outcomes after antenatal influenza immunization during the 2009 H1N1 influenza pandemic: impact on preterm birth, birth weight, and small for gestational age birth. Clin Infect Dis 2013;56:1216-22.
(22) Nordin J er al. Maternal safety of trivalent inactivated influenza vaccine in pregnant women. Obstet Gynecol 2013;121:519-25.
(23) Kharbanda E et al. Inactivated influenza vaccine during pregnancy and risks for adverse obstetric events. Obstet Gynecol 2013;122:659-67.
(24) Nordin J et al. Maternal influenza vaccine and the risks for preterm or small for gestational age birth. J Pediatr 2014;164:1051-57.
(25) Naleway et al. Safety of influenza vaccination during pregnancy: a review of subsequent maternal obstetric events and findings from two recent cohort studies. Vaccine 2014;32:3122-27.
(26) Thompson M et al. Effectiveness of seasonal trivalent influenza vaccine for preventing influenza virus illness among pregnant women: a population-based case-control study during the 2010-2011 and 2011-2012 influenza seasons.. Clin Infect Dis 2014;58:449-57.
Last updated 8/2014