The polio vaccine did not contain chimpanzee cells

However, three independent studies published in the journal Nature have cast serious doubts over the controversial theory.

A team from the National Institute for Biological Standards and Control (NIBSC) in the UK has analysed DNA from frozen samples of the suspect vaccine.

It found no genetic material from the HIV virus, or from chimpanzees.

In a second study, scientists from the Pasteur Institute in Paris, France, carried out a similar analysis and found that only cells from macaque monkeys were present in the vaccine samples.

This suggests that macaques and not chimpanzees were used in the vaccine development - and macaques do not carry SIV.

Writing in Nature, the NIBSC team, led by Dr Neil Berry, said: "Failure to detect HIV/SIV sequences or chimpanzee cellular components provides no support for the hypothesis that these materials were responsible for entry of HIV into humans and the source of Aids."

HIV subtypes

Macaque monkeys do not carry HIV-type viruses

Supporters of the polio vaccine theory point to the fact that clusters of HIV subtypes seemed to have appeared at exactly the same time.

They say this can only be explained by a simultaneous transfer of multiple viruses from chimpanzees to humans via a contaminated vaccine.

However, this argument has also been debunked by scientists at Oxford University, UK, and the Laboratoire Retrovirus in Montpellier, France, who studied the genetic diversity of HIV in the Congo - the place thought most likely to be the origin of the virus.

They found the Congo strains had sufficient variation to have produced all the different types of HIV now found around the world.

The findings indicate that HIV subtypes could have resulted from the chance exportation of Congo strains to different locations, rather than wholesale transmission from chimps.

Neil Berry carried out the research

The scientists say it is likely that the virus was transferred from chimpanzees to humans in a natural way.

For instance, humans could have become infected by eating contaminated meat, or by getting cut while out hunting.

Dr Eddie Holmes, a researcher at Oxford's Department of Zoology, said: "Although it is unlikely that we ever know exactly how HIV was transferred from chimpanzees to humans, our results certainly make the polio vaccination theory far less likely."

Writing in Nature, Dr Robin Weiss, of the Wohlvirion Centre in London, UK, concludes: "Some beautiful facts have destroyed an ugly theory."