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Questions & Answers

Thimerosal in Seasonal Influenza Vaccine

What is thimerosal?

Thimerosal is a very effective preservative that has been used since the 1930s to prevent contamination in some multi-dose vials of vaccines (preservatives are not required for vaccines in single dose vials). Thimerosal contains approximately 49% ethylmercury. There is no convincing evidence of harm caused by the low doses of thimerosal in vaccines, except for minor reactions like redness and swelling at the injection site. However, in July 1999 the Public Health Service (PHS) agencies, the American Academy of Pediatrics (AAP), and vaccine manufacturers agreed that thimerosal should be reduced or eliminated in vaccines as a precautionary measure.

Today, all routinely recommended licensed pediatric vaccines that are currently being manufactured for the U.S. market, with the exception of influenza vaccine, contain no thimerosal or only trace amounts. Thimerosal preservative-free influenza vaccines are available, but in limited quantities. The total amount of inactivated influenza vaccine available without thimerosal as a preservative will continue to increase as manufacturing capabilities are expanded.

Does the influenza vaccine contain thimerosal?

Yes, the majority of influenza vaccines distributed in the United States currently contain thimerosal as a preservative. However, some contain only trace amounts of thimerosal and are considered by the Food and Drug Administration (FDA) to be preservative-free. Manufacturers of preservative-free flu vaccine use thimerosal early in the manufacturing process. The thimerosal gets diluted as the vaccine goes through the steps in processing. By the end of the manufacturing process there is not enough thimerosal left in the vaccine to act as a preservative and the vaccine is labeled "preservative-free".

How much thimerosal-free influenza vaccine is expected to be available for the 2008-09 season?

For the 2008-09 season, manufacturers project producing as many as 50 million doses of thimerosal-free or preservative-free (trace thimerosal) influenza vaccine. Of these 50 million doses, approximately 20 million doses represent flu vaccine products that are licensed for use in children over 3 years of age.

Will the supply of thimerosal-free and preservative-free (trace thimerosal) influenza vaccine be adequate for the recommended pediatric priority groups (ages 6-59 months) during the 2008-09 season?

For the 2008-09 season, there is one product licensed for 6-23 month old children (the product is thimerosal-free). For children between the ages of 2 and 5 years of age, there are three products available that are thimerosal-free (sanofi's Fluzone; MedImmune's FluMist) or preservative-free (trace thimerosal- [Novartis's Fluvirin]). Specific information about these products and other influenza vaccines can be found in the Table: Influenza Vaccine Manufacturers for the 2008-09 Influenza Season. Given the uptake of influenza vaccine among children less than 5 years of age, and the anticipated increase in vaccination coverage for this season, CDC projects that the vaccine supply for this age group will be adequate to meet demand.

Is it safe for children to receive an influenza vaccine that contains thimerosal?

Yes. There is no convincing evidence of harm caused by the small amount of thimerosal in vaccines, except for minor effects like swelling and redness at the injection site due to sensitivity to thimerosal. Most importantly, since 1999, newly formulated thimerosal preservative-free childhood vaccines (Hepatitis B, Hib, and DTaP) have been licensed. With the newly formulated childhood vaccines, the maximum total exposure during the first six months of life will now be less than three micrograms of mercury. Based on guidelines established by the FDA, the Environmental Protection Agency (EPA) and the Agency for Toxic Substances and Disease Registry (ATSDR), no child will receive excessive mercury from childhood vaccines regardless of whether or not their flu shot contains thimerosal as a preservative.

Research suggests that healthy children under the age of 2 are more likely than older children and as likely as people over the age of 65 to be hospitalized with flu complications. In addition, children between 24-59 months of age have higher rates of influenza-related doctor and Emergency Department visits than older children. Therefore, vaccination with either reduced or standard thimerosal-content flu vaccine is recommended for children between the ages of 6 and 59 months by CDC’s Advisory Committee on Immunization Practices.

Is it safe for pregnant women to receive an influenza vaccine that contains thimerosal?

Yes. A study of influenza vaccination examining over 2,000 pregnant women demonstrated no adverse fetal effects associated with influenza vaccine. Case reports and limited studies indicate that pregnancy can increase the risk for serious medical complications of influenza. One study found that out of every 10,000 women in their third trimester of pregnancy during an average flu season, 25 will be hospitalized for flu related complications.

Additionally, influenza-associated excess deaths among pregnant women have been documented during influenza pandemics. Because pregnant women are at increased risk for influenza-related complications and because a substantial safety margin has been incorporated into the health guidance values for organic mercury exposure, the benefits of influenza vaccine with reduced or standard thimerosal content outweighs the theoretical risk, if any, of thimerosal.


Ball LK, Ball R, Pratt RD. An assessment of thimerosal use in childhood vaccines. Pediatrics 2001;107(5):1147-54.

Centers for Disease Control and Prevention. Prevention and control of influenza; recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR 2002; 51(RR03):1-31.

Centers for Disease Control and Prevention. Recommendations regarding the use of vaccines that contain thimerosal as a preservative. MMWR 1999;48(43):996-8.

Heinonen OP, Shapiro S, Monson RR, Hartz SC, Rosenberg L, Slone D. Immunization during pregnancy against poliomyelitis and influenza in relation to childhood malignancy. Int J Epidemiol 1973;2:229–35.

Izurieta HS, Thompson WW, Kramarz P, et al. Influenza and the rates of hospitalization for respiratory disease among infants and young children. New Engl J Med 2000;342:232–9.

Kirshon B, Faro S, Zurawin RK, Samo TC, Carpenter RJ. Favorable outcome after treatment with amantadine and ribavirin in a pregnancy complicated by influenza pneumonia: a case report. J Reprod Med 1988;33:399–401.

Neuzil KM, Wright PF, Mitchel EF, Griffin MR. Burden of influenza illness in children with asthma and other chronic medical conditions. J Pediatr 2000;137:856–64.

Neuzil KM, Reed GW, Mitchel EF, Simonsen L, Griffin MR. Impact of influenza on acute cardiopulmonary hospitalizations in pregnant women. Am J of Epidemiol 1998;148(11):1094-102.

Shahab SZ, Glezen WP. Influenza virus. In: Gonik B, ed. Viral diseases in pregnancy. New York, NY: Springer-Verlag, 1994:215–23.

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