Scientific evidence alone cannot prove liability in criminal HIV transmission cases

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A briefing paper published today jointly by NAM and the National AIDS Trust (NAT) explains why criminal investigations of alleged sexual HIV transmission cannot be proved conclusively by scientific evidence alone. HIV Forensics: The use of phylogenetic analysis as evidence in criminal investigation of HIV transmission is aimed at virologists and other potential expert witnesses as well as people working in the criminal justice system, but may also be useful for those supporting HIV-positive individuals as either potential complainants or defendants.

(Information on how to download or order copies of this paper are at the bottom of the page)

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.


phylogenetic analysis

The comparison of the genetic sequence of the virus in different individuals in order to determine the likelihood that two or more samples are related. This involves creating a hypothetical diagram (known as a phylogenetic tree) that estimates how closely related the samples of HIV taken from different individuals are. Phylogenetic analysis is not a reliable way to prove that one individual has infected another, but may identify transmission clusters, which can be useful for public health interventions.


A variant characterised by a specific genotype.


ribonucleic acid (RNA)

The chemical structure that carries genetic instructions for protein synthesis. Although DNA is the primary genetic material of cells, RNA is the genetic material for some viruses like HIV.



A unit of heredity, that determines a specific feature of the shape of a living organism. This genetic element is a sequence of DNA (or RNA, for viruses), located in a very specific place (locus) of a chromosome.


The study of the causes of a disease, its distribution within a population, and measures for control and prevention. Epidemiology focuses on groups rather than individuals.

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’. This is because HIV, unlike human DNA samples or fingerprints, is rapidly evolving.

Acceptable standards

The paper details the limitations, and potential pitfalls, of using phylogenetic analysis as forensic evidence. In addition, the briefing paper makes several recommendations about the way that expert witnesses should carry out phylogenetic analysis for HIV forensic purposes, as well as how the results are interpreted.

One of the key points is that it is vitally important that phylogenetic analysis for HIV forensic purposes compares the samples provided by the person being investigated and the complainant with the right kind of comparison samples (controls), otherwise this could exaggerate how related they appear to be.

To provide expert witnesses with the highest level of confidence, these comparison samples should come from around 30 other HIV-positive people in the area where the person being investigated and complainant live and/or socialise, and should ideally be collected around the time of the alleged transmission event.

However, even with the 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).
  • Both the complainant and person under investigation were independently infected with similar viral strains by a person or people from the same transmission network.

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 – neither of which have yet been used in criminal HIV transmission cases and which are both very costly and time-consuming – would be necessary to produce data relevant to this question.

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.

Legal briefing

In addition, the paper includes the legal background to criminal HIV transmission prosecutions; how phylogenetic analysis has been used in other cases around the world; and the weight that ‘expert witness’ testimony has, with a focus on English and Welsh law.

Another of the paper’s authors is Matthew Weait, of the Research Institute for Law, Politics and Justice at Keele University, who is also a member of the Department of Health’s Expert Advisory Group on AIDS (EAGA) .

"The problems that HIV transmission cases pose for the criminal law are many,” notes Dr Weait. “This briefing paper provides an invaluable and accessible explanation of the science of phylogenetic testing, and will ensure that judges, prosecutors, lawyers and medical professionals involved in cases concerning transmission are better informed, and so better able to protect the due process rights of defendants."

The NAM/NAT Briefing Paper, HIV Forensics: The use of phylogenetic analysis as evidence in criminal investigation of HIV transmissionis available as a pdf download from NAM or from The National AIDS Trust, whose webpage also includes details of how to order a printed version of the paper.