Thimerosal in Influenza Vaccines

Q:  What is thimerosal?

A: Thimerosal is a mercury-containing preservative that has been used for decades in the United States to prevent contamination in some vaccines. Preservatives prevent the growth of bacteria and fungi in the event that they get into the vaccine. Contamination by germs in a vaccine could cause serious illness or death.

Q:  Does the H1N1 influenza vaccine contain thimerosal?

A: The H1N1 flu vaccine is being made in formulations with and without thimerosal. Nasal spray formulation and some injectable vaccine do not contain thimerosal. Most of the H1N1 vaccine received by Marin County has contained the thimerosal preservative.

Q: Does thimerosal cause autism?

A: There is no conclusive evidence that any vaccine or vaccine additive increases the risk of developing autism or any other behavior disorder. Rather, evidence is accumulating of lack of harm resulting from exposure to vaccine containing thimerosal as a preservative. Numerous studies have found no association between thimerosal exposure and autism. In a 2004 report, the Institute of Medicine (IOM) concluded that there is no association between autism and vaccines that contain thimerosal as a preservative.

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

A: 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. 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. Since 1999, newly formulated thimerosal preservative-free childhood vaccines (Hepatitis B, Hib, and DTaP) have been licensed. 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. 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 children under the age of 4 are more likely than older children and and adults to be hospitalized with flu complications, and they should be vaccinated. Considering the risks of infection and the benefits of vaccination and the lack of sufficient thimerosal-free flu vaccine, the Governor of California has issued an exemption so that pregnant women and children under 3 years of age can receive H1N1 flu vaccine containing thimerosal. The exemption is valid through September 2010.

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

A: Yes. AA 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. Pregnant women are at particular risk of complications from H1N1 flu.

Because pregnant women are at increased risk for influenza-related complication, the benefits of influenza vaccine with or without thimerosal outweigh the risk, if any, of thimerosal.