(Brian Martin, submission to Nature, May 2001, not published)
The theory that AIDS arose out of contaminated oral polio vaccines [1-3] has recently received publicity that goes well beyond what is warranted by new evidence. Contrary to some headlines, polio vaccines have not yet been "exonerated." 
On the scientific front, three studies report tests of surviving vaccine samples, each modestly concluding only that their findings do not support the OPV theory.[5-7] It remains quite possible, though, that the vaccines actually used in Africa were cultured on different, contaminated primate cells.
A fourth study reports a theoretical assessment of HIV-1 phylogeny, with findings seemingly incompatible with the OPV theory. Since there have been a number of theoretical studies of HIV-1's origin, with differing conclusions, this is far from a definitive result.
Yet a typical media report states that "Four new studies essentially refute the [OPV] theory." Researcher Edward C. Holmes is quoted as saying "There is not one piece of hard evidence in favor of the polio vaccination theory."
Research in the history and sociology of science shows that in practice theories are not rejected on the basis of a single piece of evidence, since theories can be modified and evidence may later be shown to be incorrect. Indeed, in 1992 the OPV theory was prematurely rejected on the basis of evidence that was later shown to be false.
The scientific controversy over the origin of AIDS has long exhibited a striking asymmetry. The OPV theory is expected to pass demanding tests, whereas the competing theory of "natural transfer", involving for example a hunter getting monkey blood in a cut, is given only cursory scrutiny. The OPV theory is accorded no flexibility in the face of new evidence, whereas the natural transfer theory is left vague and adaptable, with little opportunity for falsification. Finally, some scientific specialists assume they can reach a conclusion about the origin of AIDS without examining relevant historical, archival and interview evidence.
Though exactly what constitutes "hard evidence" is a matter of debate, it could easily be said, to paraphrase Holmes, that "There is not one piece of hard evidence in favour of the natural transfer theory." But scientists, perhaps concerned to protect the reputation of vaccination, seem disinclined to apply the same standards of proof to the natural transfer theory that they apply to the OPV theory.
1. Curtis, T. Rolling Stone 626, 54-61, 106-108 (19 March 1992).
2. Cribb, J. The White Death (Angus & Robertson, Sydney, 1996).
3. Hooper, E. The River: A Journey Back to the Source of HIV and AIDS (Penguin, London, 2000).
4. Weiss, R.A. Nature 410, 1035-1036 (2001).
5. Blancou, P. et al. Nature 410, 1045-1046 (2001).
6. Berry, N. et al. Nature 410, 1046-1047 (2001).
7. Poinar, H. et al. Science 292, 743-744 (2001).
8. Hooper, E. Phil. Trans. R. Soc. Lond. B (in the press).
9. Rambaut, A. et al. Nature 410, 1047-1048 (2001).
10. Brown, D. Washington Post (26 April 2001).
11. Collins, H. M. & Pinch, T. The Golem: What You Should Know about Science (Cambridge University Press. Cambridge, 1998).
12. Zhu, T. & Ho, D.D. Nature 374, 503-504 (1995).
13. Martin, B. Phil. Trans. R. Soc. Lond. B (in the press).
Science,Technology & Society, University of Wollongong, NSW 2522, Australia