Influenza Vaccine Q&A

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What is thimerosal?

Thimerosal is a very effective preservative that has been used since the 1930s to prevent contamination in some multidose vials of vaccines (preservatives are not required for vaccines in single dose vials). Thimerosal contains about 49 percent 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'.

Is influenza vaccine that does not contain thimerosal as a preservative available this flu season (2006-07)?

At the current time, Sanofi Pasteur is projecting that 8 million to 9 million doses of thimerosal-free vaccine in pre-filled syringes or vials will be produced for the 2006-07 influenza season. The majority of this vaccine will be in 0.25 mL syringes (indicated for ages 6-35 months) with the remainder in 0.5 mL vials or syringes (indicated for ages 36 months and older). In addition, GlaxoSmithKline’s influenza vaccine for adults 18 and older is preservative-free vaccine and Novartis (formerly Chiron) has a preservative-free preparation for people 4 and older. Also, the nasal-spray influenza vaccine (sold commercially as FluMist) does not contain any thimerosal and can be given to healthy people 5 to 49 years of age who are not pregnant.

Will the supply of thimerosal-free and thimerosal-reduced influenza vaccine be adequate for the current and newly recommended pediatric priority groups (ages 6 to 59 months) during the 2006-07 season?

For the 2006-07 season, CDC projects that thimerosal-free vaccine supplies will be adequate for children ages 6 to 23 months. There also is likely to be sufficient preservative-free influenza vaccine for 4-year-olds. Thimerosal-free vaccine doses licensed for 3-year-olds, however, are limited in supply, and CDC anticipates that there will be insufficient vaccine for this age group. Thimerosal-containing vaccine also can be used to vaccinate children if the product’s age indication is appropriate.

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 3 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 their flu shot contains thimerosal as a preservative.

Recent research suggests that healthy children younger than 2 are more likely than older children and as likely as people older than 65 to be hospitalized with flu complications. Therefore, vaccination with reduced or standard thimerosal-content flu vaccine is encouraged when feasible in children, including those who are 6 to 23 months old.

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

Yes. A study of influenza vaccination examining more than 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.

Source: Centers for Disease Control and Prevention