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.