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    Pregnancy and the flu

    Pregnant women, even those in perfect health, can become very ill if they get the flu.

    For example, one large study examined the effect of the flu on pregnant women over the course of 17 flu seasons(1). The study found that pregnant women in the third trimester 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 the farther along the women were in their pregnancies. Healthy women at 37 to 42 weeks were almost five times as likely to be admitted to the hospital during the flu season for heart or lung problems as women who were one to six months postpartum. Another study showed that pregnant women with asthma were at extra high risk for hospitalization during the flu season(2). The pandemic H1N1 flu virus of 2009 proved to be particularly harmful to pregnant women, and it is still circulating.(3)

    Because of these findings, the U.S. Centers for Disease Control and Prevention (CDC) recommend that women who are or will be pregnant during the flu season get the flu vaccine(4). The American College of Obstetricians and Gynecologists (ACOG) concurs with this recommendation.(5)

    Will the flu vaccine help?
    Most experts believe that the flu vaccine will provide protection against the flu for pregnant women and possibly their infants as well. Despite earlier observational research suggesting little if any benefit from the flu vaccine in pregnant women(6), an experimental study from 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 new 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 to be 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. While the numbers of patients in these studies are relatively small, the results are reassuring.

    Thimerosal-free seasonal flu vaccine will be available for pregnant women at all PAMF locations as mandated by the State of California. Pregnant women should not receive the live, intra-nasal flu vaccine.

    What do physicians think?
    The vast majority of experts, including the American College of Obstetricians and Gynecologists, believe the flu shot is safe 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.


    (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. 2004. Influenza vaccination and treatment during pregnancy. ACOG Committee Opinion No. 305, November 2004. Obstetric Gynecology, 104 (5 Pt 1):1125-6.

    (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.