Pregnant women, even ones who are healthy, can have medical complications from the flu. A large study of influenza-related illness during pregnancy was reported in 1998⁽¹⁾. This study examined the effect of the flu on pregnant women during 17 previous influenza seasons.

The study found that during the flu season, pregnant women in the third trimester were just as likely to be hospitalized for heart or lung problems as women with serious, chronic medical conditions who were not pregnant. The risk increased the farther along they were in their pregnancies. Healthy women at 37 to 42 weeks of gestation 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. Pregnant women with asthma are at particularly high risk for hospitalization during the flu season⁽²⁾.

Because of these findings, the CDC now recommends that women who will be pregnant during the flu season get the flu shot⁽³⁾. The American College of Obstetricians and Gynecologists (ACOG) concurs with this recommendation⁽⁴⁾. Check your calendar. If your pregnancy overlaps with the flu season (usually late November through March), you should get a flu shot. This is especially the case if you'll be in the second or third trimester (past 13 weeks) during the flu season.

Will the flu shot help?

It is reasonable to believe that the flu shot will provide protection against the flu in pregnant women (as it does in healthy adults) though previous research did not confirm this⁽⁵⁾. However, a recent study from Bangladesh suggests protection from the flu for both the mother and infant when the mother received the flu shot⁽⁶⁾.

Is the flu shot safe in pregnant women?

Since the flu shot is an inactivated vaccine (containing 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⁽⁷⁾. A similar but smaller study showed no harmful effects of the flu shot on the fetus or the mother⁽⁸⁾. More recent research showed no serious adverse effects from the flu shot in the perinatal period or during the first six months of infant life⁽⁹⁾. While the numbers of patients in these studies are relatively small, these results are reassuring.

Thimerosal, a preservative that contains a minute amount of mercury in compound form, is present in the standard flu vaccine (the classic "flu shot"). It is considered to be a safe vaccine component by the CDC (see Thimerosal in Seasonal Influenza Vaccine). Nonetheless, a California state law effective July 1, 2006, mandates the use of influenza vaccine that has zero or trace amounts of mercury in two patient groups:

• Children under 36 months of age


• Pregnant women

PAMF will provide thimerosal-free vaccine to patients who are 6 to 35 months of age or pregnant when they present for vaccination.

What do physicians think?

Many experts (including the American College of Obstetricians and Gynecologists) believe the flu shot may be given in all three trimesters of pregnancy. Some physicians prefer to wait until after the first trimester. If you are pregnant and would like to discuss the flu shot further, please consult your primary care provider, obstetrician or one of PAMF's flu shot clinic consultants.

References

(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) Centers for Disease Control and Prevention. "Prevention and control of influenza: recommendations of the Advisory Committee on Immunization Practices (ACIP)." MMWR 2008; 57 (No. RR-7): 1-60.

(4) 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.

(5) 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.

(6) 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.

(7) 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.

(8) 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.

(9) 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.