The officials say that the very slight chance that someone could be infected is far outweighed by the benefits that these vaccines bring in fighting disease and preventing death. Creutzfeldt- Jakob disease. No one is known to have contracted the disease this way. But if we have the ability to bring this remote risk to zero, that is something we want to do. The F.D.A. so far has only investigated the vaccine makers and has not looked to see whether other medicine is free of possible mad cow contaminants.
Some experts say they worry more about dietary supplements.
Unlike drugs, supplements are largely unregulated. The F.D.A. is not even sure how many supplement makers there are. The potential exists for abuse. All five vaccine makers, which include GlaxoSmithKline, Aventis and American Home Products, have now agreed to stop using the suspect materials, which include blood, fetal calf serum and meat broth. But it will take a year or more to replace existing supplies with reformulated products, because it can take many months to grow cultures used in making vaccines.