Two expert virologists – Anne-Mieke Vandamme of Belgium’s Rega Institute for Medical Research, and Anna Maria Geretti of the Department of Virology, Royal Free Hospital, London – are co-authors of the paper, which focuses on how and why phylogenetic analysis cannot be used as the main, or only, proof that one person infected another in prosecutions for HIV transmission.
Dr Geretti served as an expert witness for the defence in the first – and so far only – criminal HIV transmission trial to end with a ‘not guilty’ verdict. The case collapsed after Dr Geretti explained the limitations of the phylogenetic analysis evidence presented by the prosecution as ‘proof’ that the defendant infected the complainant.
Phylogenetic analysis is a complex scientific process used in molecular epidemiology. An individual strain of HIV can be examined in great detail by analysing its genetic code (RNA). By examining very small differences in different parts of HIV’s RNA (obtained via gene sequencing), it is possible estimate how these HIV strains are genetically related. This involves the use of computational tools to create a hypothetical diagram (known as a phylogenetic tree). However, this method is unable to create a definitive ‘match’ because HIV, unlike human DNA samples or fingerprints, is rapidly evolving.
Even with appropriate comparison samples, phylogenetic analysis cannot ‘prove’ that HIV transmission occurred directly between two individuals. Other possibilities may include:
- The complainant was infected with a similar viral strain by someone from the same transmission network (i.e. individuals that have had sex partners in common, whether or not they are aware of this).
Even if phylogenetic analysis suggests that the two viruses are very closely related, this does not provide enough information to know the direction or timing of the alleged transmission (i.e. who might have infected who; or who might have been infected first). Additional detailed samples and complex full genome analysis would be necessary to produce relevant data.
Nevertheless, if phylogenetic analysis is carried out rigorously, it is reliable enough to show that the virus from the person under investigation and the complainant are not closely related to each other. In other words, phylogenetic analysis can exonerate the person being investigated.
The NAM/NAT Briefing Paper –
HIV Forensics: The use of phylogenetic analysis as evidence in criminal investigation of HIV transmission
Link to Aidsmap article