PARIS, Dec 6, 2006 (AFP) - A new study published in a top science journal says that six foreign medical workers, charged in Libya with deliberately infecting hundreds of children with the AIDS virus, are innocent.
The paper appears in the peer-reviewed British science journal Nature on Thursday ahead of the verdict in their retrial, expected on December 19.
They potentially face the death penalty if found guilty.
An international team of scientists analysed the genetic ID of the virus that infected the Libyan children and used a "molecular clock", based on the pathogen's rate of mutation, to estimate when the microbe could have been introduced in Benghazi's al-Fateh hospital.
It concludes this strain of human immunodeficiency virus (HIV) entered the hospital before March 1998, which was when the five Bulgarian nurses and the Palestinian doctor began to work there, the study says.
"Irrespective of which (computer) model was used, the estimated date of the most common recent ancestor for each cluster pre-dated March 1998, sometimes by many years," says the study, lead authored by Oliver Pybus of Oxford University.
"In most analyses, the probability that the clusters from the al-Fateh Hospital originated after that time was almost zero."
It says the likeliest cause of the infection was "nosocomial transmission" -- in other words, the children were contaminated by syringes or other intravenous equipment that had been used before but not been properly sterilised.
The "molecular clock" technique is commonly used by virologists to establish the family tree of an outbreak.
It is based on the premise that a virus replicates sloppily -- when it reproduces it acquires a mutation in its genetic code. These mutations accumulate over time, becoming a telltale of viral evolution.
A total of 418 children were found to be infected with HIV-1 and hepatitis C.
Of these, 248 were referred to European hospitals. The scientists sequenced viral samples from 51 children.
They identified the AIDS virus as being from the CRF-02 AG clade, or subtype, of HIV-1.
This clade was first spotted in West Africa -- and the viral samples found in Benghazi indicated it had only slightly mutated from that form, which implies it was brought in directly from that region.
A similar analysis of the hepatitis C virus found a lineage with a strain in Egypt and also to strains in Cameroun.
This linkage with sub-Saharan Africa and the Middle East "is to be expected, given the large number of migrants passing within or passing through Libya," the paper suggests.
The case of the six healthworkers has drawn heavy fire from the international community of doctors and scientists.
In November, 114 Nobel prizewinners jointly called for the trial's outcome to be based on "strong scientific evidence" and Nature, as well as its US counterpart Science and the British health journal The Lancet, also published strong appeals.
A 2003, two international specialists, Luc Montagnier, who co-discovered the AIDS virus, and Vittorio Colizzi, were asked by the Libyan authorities to conduct a scientific inquiry into the Benghazi outbreak.
They concluded that the hospital infections were probably the result of poor hygiene, but this finding was rejected by the court as being too vague and lacking in evidence.
In December 2003, a second scientific study, this time by three Libyan experts, concluded the children had been deliberately infected, given the significant number of contaminations.
This document is the centre piece of the prosecution's case against the six.
The first verdict in the trial, issued in May 2004, was the death penalty by firing squad. Libya's supreme court ordered a retrial following an appeal in December 2005.
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