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Am Fam Physician. 2008;78(11):1248

See related Practice Guideline on page 1301.

Many parents are concerned about vaccine safety. This concern is heightened by a widespread belief that mercury may be contributing to the increasing rates of autism diagnoses in children. This specific concern is not supported by the evidence.

The controversy over mercury and the measles, mumps, and rubella (MMR) vaccine dates back to 1999. Trace amounts of mercury are present in the preservative thimerosal, which was used in childhood vaccines. Thimerosal has been used as an additive in biologics and vaccines since the 1930s and has no known adverse effects. Despite good evidence of its safety, concerns were raised that the increasing number of recommended vaccines could lead to higher cumulative mercury doses. Therefore, in 1999, the American Academy of Pediatrics and the U.S. Public Health Service recommended that childhood immunizations be manufactured without the preservative. The last thimerosal-containing childhood vaccines expired in 2003.1 Because more recent evidence has confirmed the safety of thimerosal, it is still used in some formulations of influenza vaccine. However, alternative vaccine preparations without thimerosal are available.2

Many private and government organizations have determined that there is no association between vaccines and autism.35 However, fears were rekindled when the federal government agreed to compensate a child with a rare mitochondrial disorder. The child developed a regressive encephalopathy with autistic-like features two days after receiving vaccines.

When counseling parents about vaccine safety, physicians can say unequivocally that there is no association between vaccines and autism. Vaccines prevent devastating illnesses and are recommended for all children. The most compelling information to provide parents who doubt the evidence is that rates of autism spectrum disorder diagnoses in children continued to increase even after thimerosal was removed from vaccines.6

In a recent study of 3,924 parents, the most commonly cited reasons for declining vaccines were concerns about their safety and side effects.7 The varicella and MMR vaccines generated the most concern. Information or assurances from health care professionals were the main reason parents changed their minds. Reviewing authoritative sources of information with parents and giving them time to study the research can be helpful.

A discussion of common misconceptions about vaccines, including accurate information to address these concerns, is available on the Centers for Disease Control and Prevention's Web site ( For patients who distrust government-sponsored information, the Immunization Action Coalition is a nonprofit agency with reliable and comprehensive information on vaccines. Information for patients on how to evaluate online health information is available at

Concern about vaccine safety may be due to concerns parents have about their child's development. Information on identifying children with autism spectrum disorders can be found in the Practice Guideline in this issue of American Family Physician.8

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