makers, and the public prompts skepticism and even alarm that can lead to reduced vaccine use in the absence of scientific consensus.

Recent Safety Concerns. When safety claims emerge, organizations responsible for formulating recommendations that guide the use of vaccines must determine whether the reported evidence warrants the suspension or removal of a vaccine, further study, or no action. Three recent incidents—involving concerns about the safety of rotavirus vaccine, the use of thimerosal, and the reported connections between measles, mumps, and rubella (MMR) vaccine and the onset of autism—illustrate the range of issues and responses that arise in vaccine safety discussions.

In July 1999, the Advisory Committee on Immunization Practices (ACIP) and the American Academy of Pediatrics (AAP) recommended that physicians temporarily suspend administration of rotavirus vaccine. This recommendation emerged after CDC had investigated reports of several cases of intussusception (an intestinal disorder that sometimes requires surgical intervention) during the first few days after the vaccine’s ingestion (CDC, 1999i). CDC is conducting an ongoing case-control study of the vaccine and expects to report results in 2000 (AAP, 1999a). In the interim, the rotavirus vaccine has been removed from the ACIP recommended schedule.

Also in July 1999, AAP issued a joint statement with the U.S. Public Health Service (PHS) alerting physicians and the public to concern about thimerosal, a mercury-containing preservative used in some vaccines to prevent bacterial contamination. Mercury can be toxic to children, depending on the form used, route of entry, dose, and age of exposure. AAP and PHS have recommended that government agencies work rapidly to diminish children’s exposure to mercury from all sources, including vaccines (AAP, 1999b). Although future research findings may lead to changes in the vaccine schedule or the use of selected products, the thimerosal warning has not modified current ACIP recommendations in the absence of alternative products.

More recently, a series of stories in the media have suggested the possibility of a connection between autism and vaccines, especially the MMR vaccine. Unlike the rotavirus and thimerosal cases, in which the weight of clinical evidence attracted the attention of medical organizations, safety concerns about possible links between vaccines and autism have emerged primarily from preliminary research findings that have drawn public and political attention. Since the MMR vaccine is administered at the same time that symptoms of autism may become apparent, some parents have concluded that the vaccine is responsible for the syndrome. Two congressional hearings on vaccine safety (U.S. House of Representatives, 1999b, 2000) provided an opportunity for testimony that at-

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