The team, headed by an American biologist Michael Worobey of the University of Arizona, says that it has direct proof to back its findings, which are to be published in the British scientific review Nature Thursday.

The theory, put forward in the 1990s, argues that the polio vaccine was contaminated by a virus carried by chimpanzees. The vaccine was given between 1957 and 1960 to about a million people in the Belgian Congo, which later became Zaire and then the Democratic Republic of Congo.

An American journalist, Edward Hooper, argued that the tissue of chimpanzees, potentially contaminated, was used in the manufacture of the vaccine.

Scientists agree that the Human Immunodeficiency Virus (HIV), which can lead to AIDS, derives from its counterpart among chimpanzees, Simian Immunodeficiency Virus (SIV), though there is no consensus on how the virus crossed the species barrier.

But the research team, made up of African, British and American researchers, found a new variant of SIV (SIVcpzDRC1) in chimpanzees which belongs to a different family tree from the virus found in chimpanzees which is related to HIV.

In the light of this discovery and of the earlier analysis of samples of vaccine from the period which show no sign of SIV, HIV or the DNA of chimpanzees the team says the theory of a link between the vaccine and AIDS should be abandoned.

"Belief that polio vaccine can spread AIDS has hampered the World Health Organisation's efforts to stamp out polio," said Nature.

"In Nigeria, several states recently banned the use of the vaccine. Nigeria now has the highest number of polio cases in the world," it added.

TERRA.WIRE