Researchers have long puzzled over the origin of AIDS. Most hold that it arose when a simian version of HIV jumped to a human, probably in West Africa in the early- to mid-20th century, and a number of theories have emerged to explain just how it made that jump. None have been proved, but the list of ideas may be getting shorter. New reports published this week in Nature and Science may finally refute one particularly controversial proposal.

The theory in question emerged in 1999, when journalist Edward Hooper published The River: A Journey to the Source of HIV and AIDS. In this book, he asserted that scientists testing a new oral polio vaccine (OPV) in Africa in the late 1950s inadvertently sparked the AIDS pandemic. He argued that the polio researchers grew the vaccine in cells taken from chimpanzees infected with the simian immunodeficiency virus (SIV) variant most closely related to HIV, which, when introduced into humans, was able to establish a lethal foothold in the new host.

Since then, four independent teams have combed the remaining stocks of the original vaccine, searching for chimpanzee tissue, HIV and SIV. But none of the stocksincluding the one used to make the OPV tested in the Congo vaccine trial that Hooper specifically blamed for touching off the spread of HIVhave yielded any such signs. The investigators did, however, find evidence of macaque monkey tissue, thus backing the OPV researchers' claims that they grew the vaccine in macaque cells. Other results fail to support arguments made by Hooper on the basis of the evolutionary tree formed by various HIV strains.

"The new data may not convince the hardened conspiracy theorist who thinks that contamination of OPV by chimpanzee virus was subsequently and deliberately covered up," Robin A. Weiss writes in a commentary accompanying the reports in Nature. "But those of us who were formerly willing to give some credence to the OPV hypothesis will now consider that the matter has been laid to rest."