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FDA Bureau of Biologics scientists investigated the presence of bacteriophages in polio, measles, and other vaccines, any possible safety concerns, and the possibility of eliminating these while preserving the integrity of the vaccines. Studies by Bureau as well as industry scientists indicated the problem in trying to inactivate a bacteriophage with heat, which required a temperature and duration so long as to destroy the growth-promoting factors of the biological. Chemical treatment proved problematical as well. Studies continued, but by 1975, FDA assayed all live virus vaccines submitted by manufacturers for bacteriophages prior to release. However, in Bureau studies of bacteriophages in a variety of animal models in late 1978, subjecting these models to phage doses of more than a billion times those found in contaminated vaccines, no adverse effects were found; histological examinations and inspections for chromosomal damage were included in the evaluation. This suggested that bacteriophages were not dangerous, even in contaminated vaccines.

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