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The controversial notion that AIDS originated from a polio vaccine has been dealt a fatal blow. Backers of the theory had claimed that SIV, an HIV-like virus which infects chimpanzees, had tainted an experimental polio vaccine given to volunteers in the 1950s in central Africa. But now Simon Wain-Hobson and Jean-Pierre Vartanian of the Pasteur Institute in Paris have established virtually beyond doubt that this didn’t happen. They have confirmed that mysterious DNA fragments in the vaccine samples came from macaque cells rather than chimpanzee tissue. Macaques can’t be infected with SIV, say the researchers in the early edition of the Proceedings of the National Academy of Sciences.