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Is the influenza vaccine safe during pregnancy?
Reviewed By Medical Advisory Board

The influenza vaccine appears to be safe for use during pregnancy.

Various studies to date have found no increase in the rate of major birth defects in the infants of women who received influenza vaccine during any trimester of their pregnancy [10-12,13,21 ]. "During 2000--2003, an estimated 2 million pregnant women were vaccinated, and only 20 adverse events among women who received trivalent inactivated influenza vaccine were reported to the Vaccine Adverse Event Reporting System (VAERS) during this time, including nine injection-site reactions and eight systemic reactions (e.g., fever, headache, and myalgias). Pregnant women are at increased risk for influenza-associated illness and death [1-6]. Women in their third trimester of pregnancy appear to be especially likely to be hospitalized for an acute cardiopulmonary illness during influenza season [7] Influenza vaccines offer an effective means for reducing cases of influenza, especially when the composition of the vaccine  matches the circulating type of influenza.  [8,9,22,23].

In addition, maternal influenza immunization has also been shown to significantly reduce the rate of influenza in the infants of vaccinated mothers and could be used  to protect vulnerable newborns against strains of flu such as H1N1 [25]. Presently the Advisory Committee on Immunization Practices (ACIP) recommends vaccination of all women who are pregnant or will be pregnant during influenza season with inactivated influenza vaccine [21].

Are the preservatives in influenza vaccines safe?

Concerns regarding the presence of the mercury-based preservative thimerosol found in the multidose influenza vaccine appear to be unfounded. The amount of mercury in each dose of influenza vaccine is about the same amount of mercury found in nine ounces of canned tuna, and is in a form that appears to be rapidly eliminated from the body.

There is no convincing evidence of harm caused by the small amount of thimerosal in vaccines, except for minor effects such as swelling and redness at the injection site due to sensitivity to thimerosal. Currently, the U.S. Environmental Protection Agency uses a reference dose of 0.1 g/kg body weight/day as a methylmercury exposure without recognized adverse effects [17]. Each dose of inactivated flu vaccine contains 25 micrograms of mercury [16]. 25 micrograms of mercury is far below the U.S. EPA reference dose for the average 155 pound (70 kg) woman, and is about the same amount of mercury found in nine ounces of canned tuna [18]. I addition the mercury in thimersol is in the form of ethylmercury which appears to be eliminated from blood rapidly via the stools after administration of thiomersal in vaccines. Interpretation Administration of vaccines containing thiomersal does not seem to raise blood concentrations of mercury above safe values in infants. Ethylmercury seems to be eliminated from blood rapidly via the stools after parenteral administration of thiomersal in vaccines [24].

Nonetheless, certain states have enacted legislation that limits the amount of mercury in vaccines administered to pregnant women and very young children. (19, 20).

Are there adjuvants  in influenza vaccines?

No. Neither the influenza A (H1N1)  vaccine nor the seasonal influenza vaccines contain adjuvants.

Adjuvants are compounds added to vaccines that stimulate the body's immune system to react to some of the viral parts that may not cause a sufficient response on their own. They could also be used to allow a smaller dose of the virus to be given in order to stretch the amount of a vaccine that is in short supply [26,27].

Online Resources:

  • Institute for Vaccine Safety
  • CDC's Vaccines & Immunizations
  • H1N1 Flu Vaccination Resources
  • Find a Flu Clinic 


    1. Harris JW. Influenza occurring in pregnant women: a statistical study of thirteen hundred and fifty cases. JAMA 1919;72:978--80.
    2. Greenberg M, Jacobziner H, Pakter J, Weisl BA: Maternal mortality in epidemic of Asian influenza. Am J Obstet Gynecol 76;897-902.1957 PUBMED
    3. Freeman DW, Barno A: Deaths from Asian influenza associated with pregnancy. Am J Obstet Gynecol 78. 1172-1175.1959;
    4 Widelock D, Csizmas L, Klein S. Influenza, pregnancy, and fetal outcome. Public Health Rep. 1963;78:1-11. No abstract available. PMID: 14000477
    5. Kort BA, et al., Fatal influenza A pneumonia in pregnancy. Am J Perinatol. 1986;3(3):179-82.PMID: 3718637
    6. Fatal swine influenza pneumonia during late pregnancy. Arch Intern Med. 1990 Jan;150(1):213-5.PMID: 2153372
    7. Neuzil KM, et al., Impact of influenza on acute cardiopulmonary hospitalizations in pregnant women.Am J Epidemiol. 1998;148:1094-102. PMID: 9850132
    8. Jefferson TO, et al. Vaccines for preventing influenza in healthy adults. Cochrane Database Syst Rev. 2007 Apr 18;(2):CD001269. Review. PMID: 17443504
    9. Zaman K et al. Effectiveness of maternal influenza immunization in mothers and infants. N Engl J Med. 2008;359(15):1555-64. Epub 2008 Sep 17. PMID: 18799552
    10. Heinonen OP, Slone D, Shapiro S: Immunizing agents. In: Kaufman DW, ed. Birth defects and drugs in pregnancy, Publishing Sciences Group Littleton, MA 1977: 314-321.
    11. Deinard AS, Ogburn , Jr. , Jr.P:  A/NJ/8/76 influenza vaccination program: effects on maternal health and pregnancy outcome.Am J Obstet Gynecol;140.240-245.1981
    12. Munoz FM, Greisinger AJ, Wehmanen OA, et al: Safety of influenza vaccination during pregnancy. Am J Obstet Gynecol 192. 1098-1106.2005
    13. Pool V, Iskander J Safety of influenza vaccination during pregnancy. Am J Obstet Gynecol. 2006 Apr;194:1200;PMID: 16580337
    14. Heron J, Golding JALSPAC Study Team: Thimerosal exposure in infants and developmental disorders: a prospective cohort study in the United Kingdom does not support a causal association. Pediatrics 114. 577-583.2004;
    15 Global Advisory Committee on Vaccine Safety: Statement on thiomersal. (accessed 11/12/2008)http://www.who.int/vaccine_safety/topics/thiomersal/statement200308/en/index.html
    16.Prevention and control of influenza: recommendations of the Advisory Committee on Immunization Practices (ACIP).MMWR Recomm Rep. 2004 May 28;53(RR-6):1-40.
    PMID: 15163927
    17. Human Exposure | Mercury | US EPA
    (accessed 11/12/2008) http://www.epa.gov/mercury/exposure.htm
    18. Yess NJ. US Food and Drug Administration Survey of Methylmercury in Canned Tuna. AOAC Int. 1993;76:36-38
    19. Gostin LO. Medical countermeasures for pandemic influenza: ethics and the law. JAMA 2006;295:554--6.
    20. California Health and Safety Code Sections 124172. Article 9.
    21. Fiore AE. et al. Prevention and control of influenza. Recommendations of the Advisory Committee on Immunization Practices (ACIP), 2007.MMWR Recomm Rep. 2007 Jul 13;56(RR-6):1-54. PMID: 17625497
    22. McNeil SA, Influenza vaccine programs and pregnancy: new Canadian evidence for immunization.J Obstet Gynaecol Can. 2007 Aug;29(8):674-6. PMID: 17714623 <
    23.Mak TK et al. Influenza vaccination in pregnancy: current evidence and selected national policies.Lancet Infect Dis. 2008 Jan;8(1):44-52. PMID: 18156088
    24. Pichichero ME, Cernichiari E, Lopreiato J, Treanor J. Mercury concentrations and metabolism in infants receiving vaccines containing thiomersal: a descriptive study. Lancet 2002;360:1737-1741. PMID: 12480426
    25. Effectiveness of maternal influenza immunization in mothers and infants. N Engl J Med. 2008;359(15):1555-1564 PMID: 18799552
    26.MMWR .August 28, 2009 / 58(RR10);1-8. Use of Influenza A (H1N1) 2009 Monovalent Vaccine
    Recommendations of the Advisory Committee on Immunization Practices (ACIP), 2009
    27. http://www.fda.gov/BiologicsBloodVaccines/Vaccines/QuestionsaboutVaccines/ucm182335.htm



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