Public Release: 

How harmful are additives and preservatives in childhood vaccines?

Review of scientific data may allay parental concerns on vaccine safety

Children's Hospital of Philadelphia

After reviewing dozens of scientific studies, a leading vaccine expert concludes that preservatives, additives and other substances contained in vaccines pose very little risk to children receiving those vaccines.

In addition to the primary ingredient that stimulates a protective immune response, various vaccines may contain small amounts of metals, proteins, and other chemicals, some of which are residual by-products of normal vaccine manufacturing. "Parents can be reassured that the trace quantities of mercury, aluminum and formaldehyde contained in vaccines will not harm their children," says the study's lead author, Paul A. Offit, M.D., chief of Infectious Diseases and director of the Vaccine Education Center at The Children's Hospital of Philadelphia.

The article, which summarizes studies of human and animal exposures, appears in the December 2003 issue of Pediatrics. Dr. Offit's co-author is pharmacist Rita K. Jew, Pharm.D., also of The Children's Hospital of Philadelphia.

Of the substances studied, say the authors, only gelatin proteins and egg proteins are known to have harmful effects while in vaccines, but such effects are very rare. Those proteins may cause severe hypersensitivity reactions in children with allergies to gelatin or eggs. Dr. Offit adds that physicians should take appropriate safeguards for children known to have those allergies. For instance, children with allergies to the egg protein found in influenza vaccine can be desensitized to the vaccine.

One controversial vaccine-related substance is thimerosal, a compound of mercury used as a preservative to prevent contamination by bacteria or fungi. It has received considerable scrutiny by Congress and the news media since its removal from most U.S. childhood vaccines in 2001. Thimerosal was removed from vaccines as a precaution, even though there has never been scientific evidence of a link between thimerosal and adverse effects.

One confusing element, say the authors, is an incorrect assumption that ethylmercury, contained in thimerosal, has the same effects as methylmercury, a more toxic mercury compound found in the environment, but not in vaccines. "An important difference is that the body eliminates ethylmercury far more quickly than it eliminates methylmercury," adds Dr. Offit.

Other substances found in vaccines, in addition to the active agent designed to stimulate a protective response, are additives, adjuvants and manufacturing residuals.

Additives, which include various sugars, amino acids and proteins, are used to stabilize vaccines during processing and storage. Except for very rare hypersensitivity reactions to gelatin proteins, vaccine additives have not been associated with disease.

Adjuvants, such as aluminum compounds, enhance the desired immune response the vaccine is designed to produce. "The safety of aluminum has been established by experience over the past 70 years with hundreds of millions of people inoculated with aluminum-containing vaccines," say the authors, who add that the small quantities of aluminum in vaccines have not been shown to be harmful in animal studies.

Substances remaining in vaccines as residuals of the manufacturing process may include antibiotics, formaldehyde, and egg and yeast proteins. With the exception of the very rare reactions caused by egg proteins, the quantities of other residual substances are too small to demonstrate any documented reactions in those receiving the vaccines.

"Based on the best available scientific evidence," concludes Dr. Offit, "parents can be confident that vaccines are overwhelmingly safe, as well as effective in preventing infectious diseases."


About the Expert:
Paul A. Offit, M.D., is the director of the Vaccine Education Center and chief of Infectious Diseases at The Children's Hospital of Philadelphia. An internationally recognized expert in virology, immunology and vaccine safety, he is a former member of the Advisory Committee on Immunization Practices to the Centers for Disease Control and Prevention. In addition to publishing more than 90 peer-reviewed scientific papers, Dr. Offit is co-author of the book "Vaccines: What Every Parent Should Know." He frequently lectures to national and international healthcare organizations about vaccine safety and efficacy.

Under the direction of Dr. Offit, The Children's Hospital of Philadelphia established The Vaccine Education Center in October 2000 to respond to the rapidly growing need for accurate, up-to-date, science-based information about vaccines and the diseases they prevent. The Center is a nationally recognized educational resource for healthcare professionals and parents, providing information on the full spectrum of vaccine-related topics. Approximately 400 people per day visit the Center's comprehensive Web site (

Founded in 1855 as the nation's first pediatric hospital, The Children's Hospital of Philadelphia is ranked today as the best pediatric hospital in the nation by U.S. News & World Report and Child magazine. Through its long- standing commitment to providing exceptional patient care, training new generations of pediatric healthcare professionals and pioneering major research initiatives, Children's Hospital has fostered many discoveries that have benefited children worldwide. Its pediatric research program is among the largest in the country, ranking second in National Institutes of Health funding. In addition, its unique family-centered care and public service programs have brought the 430-bed hospital recognition as a leading advocate for children and adolescents from before birth through age 19. For more information, visit

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