At a meeting last September, a controversial theory that a contaminated polio vaccine triggered the AIDS epidemic was all but pronounced dead. Now, four scientific papers collectively declare that--to paraphrase the Munchkin coroner in The Wizard of Oz--the theory is not only merely dead, it's really most sincerely dead.
In his hotly debated 1999 book, The River, British writer Edward Hooper linked the first known cases of AIDS to tests of an oral polio vaccine in 1 million Africans more than 40 years ago. Hooper contends that during the manufacturing process, scientists accidentally introduced a precursor of HIV, a chimpanzee virus known as SIVcpz, into the vaccine, after they grew the poliovirus vaccine in cells taken from chimps infected with SIVcpz. The scientists, led by Hilary Koprowski, former director of the Wistar Institute in Philadelphia, denied the charge, asserting that they grew the vaccine virus in monkey, not chimp, cells. They said no evidence supported the notion that SIVcpz or HIV had contaminated any batches of the vaccine.
Preliminary data presented at a Royal Society meeting in London (ScienceNOW, 12 September 2000) challenged each of Hooper's main claims, and the new papers now formally dismiss them. The authors of two papers published in Nature this week and one in Science examined old samples of Koprowski's vaccine and found that none contained DNA from chimpanzee cells. Each lab also found evidence of monkey DNA. Two of the labs also looked for genetic material from HIV or SIVcpz but found none.
The fourth paper, published in Nature by evolutionary biologist Edward Holmes and co-workers at Oxford University, analyzed an altogether different contention made by Hooper: that the odd shape of the evolutionary tree formed by different strains of HIV supports the contaminated polio vaccine theory. Hooper highlighted the fact that the various subtypes of HIV seemed to appear simultaneously, forming clusters called "starbursts"; these theoretically could have occurred if this massive human trial used an SIVcpz-contaminated vaccine. By studying 197 HIV isolates obtained in 1997 in the Congo--where the bulk of these polio vaccine tests took place--Holmes and co-workers found that the HIV tree does not show the distinctive subtypes that are seen in many previously constructed evolutionary trees.
Hooper did not respond to an interview request. But to Holmes, these studies have, in the absence of new evidence, thoroughly dismissed Hooper's theory. "Hooper's evidence was always flimsy, and now it's untenable," Holmes says. "It's time to move on."