While U.S. health officials still rely exclusively on Salk killed-virus vaccine in the fight against poliomyelitis, a dozen countries around the world are testing live-virus preparations—all developed, ironically, in the U.S. Early results are highly promising, and so far no ill effects have been reported despite the seemingly greater danger with live virus.
All live vaccines must be made with viruses that have no power to cause disease. A few occur that way in nature. Most are formerly virulent strains that have been "attenuated" by growing them in animals (or eggs) that are...
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