The Associated Press

New studies have cast more doubt on the idea that AIDS arose because an oral polio vaccine was contaminated with a precursor to the AIDS virus.

For years, there has been speculation that the polio vaccine was grown in chimpanzee kidney cell cultures that carried the precursor virus. The virus was then passed to people when the vaccine was administered in Africa in the late 1950s, the theory holds.

But in the April 26 issue of the journal Nature, British and Swedish scientists report that they found no chimp DNA in a stock of early polio vaccine used in Africa in the 1950s, challenging the hypothesized use of chimp cells.

And an evolutionary analysis of HIV strains in the Congo, reported by British and French scientists, indicates they trace back to a common ancestor that infected people rather than chimps. That argues against the idea that the strains began diversifying in chimps, followed by several transfers to people by way of polio vaccinations.

The new reports follow up on two lines of experimental results that cast doubt on the AIDS-polio theory last year.

Those tests of early polio vaccine stocks used in the African program also found no sign of chimp DNA, scientists reported at the time. And an analysis of the evolutionary tree of HIV, the AIDS virus, suggested the human virus appeared around 1930, well before the polio vaccine campaign.

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