The earliest vaccines were made from liquid from the pocks of cattle.
Hooper tries to prove that HIV spreads from chimps to man in contaminated experimental polio vaccines that were tested in Africa.
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胡珀试图证明，爱滋病病毒是人们在非洲做小儿麻症疫苗试验时因疫苗受污染由黑猩猩传染给人的。
Samples of the original vaccines still exist, but time may have degraded them beyond use.
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虽然当时的疫苗样品还在，但是相隔时间太长，疫苗可能已经失去效用。
They fear that it could erode public confidence in the nation's largely successful vaccination policies and lead to outbreaks of many infectious diseases now held in check by inoculations.
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In July the American Academy of Pediatrics and the US Public Health Service urged vaccine makers to remove the trace of mercury preservative added to many vaccines to kill bacteria.
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