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20 IMMUNIZA LION SAFETYRE:VIEW lion (FDA) regulations require that preservatives be used in multidose vials of vaccines, except live viral vaccines, to prevent bacterial and fungal contamination (General Biologics Product Standards, 2000), which can lead to serious illness and death ~ recipients (Wilson, 1967~. In addition to its use as a preservative, ~irnerosal is used as an inactivating or bacteriostatic agent In the manufacturing process for some vaccines. Uses other than as a preservative contribute little to the final concentration of thimerosal in vaccines (Ball et al., 2001~. In this report, when the committee refers to thimerosal-free vaccines, it includes vaccines that contain only traces of thimerosal (<0.5 fig Hg per dose) left over from the manu- facturing process. In 1999, FDA determined that under the recommended childhood immuni- zation schedule, infants might be exposed to cumulative doses of ethylmercury that exceed some federal safety guidelines established for ingestion of methyl- mercury, another form of organic mercury (Ball et al., 2001~. In July 1999, the American Academy of Pediatrics (AAP) and the U.S. Public Health Service (PHS) issued a joint statement recommending the removal of thimerosal from vaccines as soon as possible (CDC, 1999a). The statement also recommended a temporary suspension of the birth dose of hepatitis B vaccine for children born to low-risk mothers until a thimerosal-free alternative became available. With the licensure of a thi~nerosal-free hepatitis B vaccine in August 1999 (CDC, l999c), at least one formulation of each vaccine on the recommended childhood immunization schedule for children age six years or younger was available without thimerosal. With the FDA approval of a thimerosal free version of DTaP vaccine in March 2001, all formulations of vaccines on the recommended childhood imm~,ni~ation schedule that are given to children six years of age or younger are available thimerosal-free in the United States (CDC, 2001a). These actions do not, however, reduce the importance of bying to resolve whether the past presence of thimerosal in some vaccines could have caused neurodevelopmental problems in some children. Moreover, thimerosal remains in use in many other countries, which continue to depend on multi-dose supplies of vaccine that must be protected Tom microbial contamination. ORIGINS OF THE IMMU~TION SAFETY REVIEW PROJECT The federal government has responded to concerns about He safety of vac- cines through several mechanisms. In 1986, Congress passed the National Childhood Vaccine Injury Act (Public Law 99-660), followed by the Vaccine Compensation Amendments of 1987 (Public Law 10~203~. This legislation mandated the establishment of He National Vaccine Injury Compensation Pro- gram and the Vaccine Adverse Event Reporting System (VAERS), a national passive surveillance system The legislation also provided for the development of vaccine-information statements for parents of children receiving immun~a-

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THIMEROSAL-CONTAINING VACCINES 21 lions. These activities are managed by three agencies of the U.S. Department of Health and Human Services (DHHS): the Centers for Disease Control and Pre- vention (CDC), the FDA, and the Health Resources and Services Administration (HRSA). The compensation program is jointly administered by HRSA and the Department of Justice. The legislation also called for IOM to review evidence regarding possible ad- verse consequences of childhood immunizations. Three expert committees con- vened by IOM produced the reports Adverse Effects Following Pertussis and Ru- bella Vaccines (IOM, 1991a), Adverse Events Associated with Childhood Vaccines: Evidence Bearing on Causality (IOM, 1994a), and DPT Vaccine and Chronic Nervous System Dysfunction: A New Analysis (IOM, 1994b). Following the completion of the third study, IOM was asked to organize the Vaccine Safety Forum to provide a framework for continued discussion of vaccine safety issues. Forum participants included representatives of government agencies, advocacy groups, and pharmaceutical companies, as well as parents, health care providers, academic researchers, and IOM staff. Forum discussions, on topics such as re- search strategies and risk communication, were documented in brief reports (IOM, 1996, 1997a,b) but were not intended to produce conclusions or recommendations. In 1995 and 1997, in response to the findings and recommendations of Ad- verse Events Associated with Childhood Vaccines: Evidence Bearing on Cau- sality (IOM, 1994a), the Secretary of HHS updated the Vaccine Injury Table, a list of post-vaccination events that must be reported to DHHS and that are cov- ered by the National Vaccine Injury Compensation Program. Also in 1995, the National Vaccine Advisory Committee (NVAC) of the National Vaccine Pro- gram Office of DHHS added a Vaccine Safety Subcommittee to its efforts. In 1999, this subcommittee expanded its scope and was renamed the Vaccine Safety and Communication Subcommittee. Concern over cases of vaccine- associated paralytic poliomyelitis prompted another CDC committee the Advi- sory Committee on Immunization Practices to recommend in 1997 that the immunization schedule be changed to replace oral poliovirus vaccine with inac- tivated poliovirus vaccine (CDC, 2000a). Since the mid-199Os, additional challenges to the safety of vaccinations have gained attention in various settings. From 1999 through mid-2001, the Committee on Government Reform of the U.S. House of Representatives held eight hearings on vaccine safety issues. The media have covered these issues on news programs such as 60 Minutes, 20/20, and Nightline, and the Internet is an increasingly im- portant communication channel. Also, many consumer and professional organiza- tions have sponsored conferences and scientific symposia to address vaccine safety concerns. Given these growing concerns, CDC and the National Institutes of Health (NIH) recognized the need for an independent, expert group to address vaccine safety in a timely and objective manner. In 1999, as a result of IOM's previous work and its access to independent scientific experts, CDC and NIH began a