government agencies. Government support of these studies is an important subsidization of the vaccine development process.

On the demand side, the purchase, distribution, and administration of vaccines are carried out through a mixture of federal, state, and private sector activities. The FDA subserves the regulatory role in vaccine licensing; the CDC is a major buyer of vaccines for federal and state programs, purchasing more than half of the vaccines used for childhood immunizations in the United States. The cost to consumers of vaccines purchased by the CDC is much lower than the cost of vaccines sold by the private sector market. Policies for the use of vaccines are developed by the Immunization Practices Advisory Committee, a CDC advisory committee, within the licensed-use guidelines set by the FDA.

Many decisions influence the life cycle of a new vaccine. In the public sector, such decisions are made independently by a number of agencies or committees (e.g., the FDA, CDC, Immunization Practices Advisory Committee [ACIP]) and are loosely coordinated by the PHS through its National Vaccine Program Office. (This agency is a coordinating office for the PHS but has no directive authority.) Corporate decision making responds primarily to market forces. The relationship between the public and private sectors is defined mainly by FDA guidelines and federal purchasing regulations and, as a result, is as often confrontational as cooperative.

Advances in immunology, molecular biology, biochemistry, and drug delivery systems have stimulated major new initiatives in vaccine development. The generation of vaccines that will come into use in the next decade is likely to be different from previous generations of vaccines. Some will contain more than one highly purified antigen and will rely on new delivery methods. Programmed-release biodegradable microspheres offer the possibility of single-dose regimens for parenteral vaccines. New oral vaccination methods will improve our ability to protect against enteric and respiratory agents.

Extensive investigations are also centering on vaccines that use attenuated viruses and bacteria as vectors to introduce specific antigenic components of disease-causing microbes. For example, a fowlpox virus recombinant, which has had parts of the genome of rabies virus inserted into its DNA, has been tested in animals to determine its ability to induce immunity to rabies. In two of five vertebrate species examined in one study, inoculation of the fowlpox recombinant vaccine candidate resulted in the induction of an immune response that protected against subsequent challenge with live rabies virus (Taylor et al., 1988). Other attenuated organisms being considered for use as vaccine vectors are vaccinia virus, baculovirus, poliovirus, Salmonella typhimurium, and bacille Calmette-Guerin (BCG). A major advantage of the vectored vaccine concept is that the vector genome can accommodate genetic material from more than one agent (perhaps as many

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