Understanding HIV transmission

Last updated on: 24 April 2012

In criminal HIV exposure cases, the severity of punishment is supposed to reflect the risk facing the uninfected party, and yet, so far, with a few rare exceptions (see below), courts throughout the world are being slow to adapt to the unfolding evidence about HIV treatment reducing viral loads to undetectable levels and, consequently, lowering the risk of transmission.

Even without treatment, sexual HIV exposure leads to HIV transmission much less often than generally presumed. In fact, much more often than not, sexual exposure to HIV does not result in HIV transmission. Although it is possible for one act of unprotected sexual intercourse to transmit HIV, the risk of this happening is low and varies according to various factors, the most important of which is the viral load – i.e. the amount of HIV in the relevant bodily fluid of the HIV-positive person (i).

Other factors include:

  • the type of sexual activity (anal, vaginal or oral sex)
  • whether the HIV-positive person is the insertive or receptive partner
  • the presence (or absence) of other sexually transmitted infections in both the HIV-positive and the HIV-negative partner
  • whether (or not) the male sexual partner of an HIV-positive individual is circumcised.

Of course, the use of male or female condoms (protected sex) reduces the risk of HIV exposure and transmission so substantially – regardless of any of the above factors – that the risk is estimated to be unquantifiable. (ii)

Calculating the risk of HIV transmission through sexual contact is a difficult task because although it is possible to generalise from studies that include large numbers of people, and assess risk reasonably confidently, these data cannot be relied upon to assess individual risk, because individual risk depends on too many variables.

However, the most recently published guidelines to assess the risk of HIV transmission, per sexual exposure, estimate the following:

Type of sexual intercourse

(all without a condom)

Estimated risk of HIV transmission, per exposure (iii)

 Vaginal (risk for the woman being penetrated)  0.1%-0.2% (1 in 1000 – 1 in 500)
 Vaginal (risk for the man doing the penetration)  0.06% (1 in 1,666)
 Anal (risk for the man or woman being penetrated)  0.1%-3% (1 in 500 – 1 in 33)
 Anal (risk for the man doing the penetration)  0.06% (1 in 1,666)
 Oral (risk for the man or woman performing fellatio)  0-0.04% (0 – 1 in 2,500)

More recently, a review of all the studies on heterosexual transmission, published in August 2008 (iv), questions whether, given the all variables involved (viral load, circumcision, sexually transmitted infections) it is possible to be so precise.

Wolf and Vezina have written that a fundamental flaw in HIV exposure laws is that these laws “reflect society’s interest in protecting individuals from physical harm. Those who intentionally or recklessly expose others to HIV infection create a substantial but preventable harm. Their intentional behavior is similar to other actions prohibited by criminal statutes, such as assault and battery with fists or a weapon; thus, it would be inconsistent with the purpose of criminal law to exclude this behavior completely from its scope.” (v)

However, they argue, although each single act of HIV exposure can be punished, each single act of HIV exposure does not do the same kind of harm as assault with a fist or a weapon. A single – and often many – acts of HIV exposure may not result in the uninfected partner becoming HIV-positive.

In at least one jurisdiction, however, the per-act risk of (in this case, oral and anal) transmission is now not considered to be significant enough to prosecute even when HIV transmission has occurred without prior disclosure. “Unprotected sexual contact by people with HIV may be accompanied by the possibility of the transmission of HIV,” wrote the Supreme Court of the Netherlands in a January 2005 ruling, “but this does not mean that the probability of this is appreciable – apart from exceptional risk-exacerbating circumstances.” (vi)

The Swiss statement: changing the rules?

In January 2008, a consensus statement from the Swiss National AIDS Commission (EKAF) (vii) said that, as long as someone has had an undetectable viral load (which they defined as less than 40 copies/ml) for at least six months; remains adherent to their HIV treatment; is evaluated regularly by their doctor; and has no other sexually transmitted infections; then they are “not sexually infectious, i.e. cannot transmit HIV through sexual contact.”

Although the idea that effective treatment has a major impact on transmission on a population (rather than an individual) level is not new, HIV experts had never before make a public statement on the effect of treatment on transmission on an individual level.

Since then, other experts from around the world have discussed this (see, for example, this report from a meeting at the International AIDS Conference in Mexico) and Australasian experts have rejected the statement, while agreeing that the per-act risk is incredibly low. Another set of experts note that, “Denying an effect of treatment on risk of transmission would be dishonest and futile...” (viii) Consequently, jurisdictions that have HIV exposure laws do need to rethink their definitions of HIV exposure, and debate whether the punishment fits the ‘crime’.

Can the Swiss statement, and the notion of treatment’s effect on transmission risk, be used as a defence for people who are accused of exposure when on a stable regimen with an undetectable viral load? Two recent court cases suggest it can.

In May 2008, the US Court of Appeals for the Armed Forces spent some time discussing whether an HIV-positive soldier who had previously pleaded guilty to criminal HIV exposure charges could set aside his guilty plea following testimony from a military doctor that he was "highly unlikely" to be able to transmit HIV given his extremely low viral load. Although the majority did not agree, and did not allow his guilty plea to be set aside, two members of the appeals panel found this expert testimony valid enough to question HIV exposure laws given evolving scientific knowledge of HIV transmission, and said that if the case had been an appeal of a verdict, they would have quashed the conviction.

In July 2008, Queen's Bench Justice Joan McKelvey, who presided over the trial of Clato Mabior in Winnipeg, Canada, seriously considered up-to-date science on the effect of HIV treatment on the risks of HIV exposure. However, she wasn’t convinced. “I have found the medical and scientific evidence to be very persuasive that even with an undetectable viral load, there remains a risk of transmission of HIV with resultant endangerment of life,” she said. “This is particularly so given the medical evidence that other influences or factors such as STDs or the use of female contraception can affect or ‘spike’ a viral load.” She also noted that the Swiss statement – and the publicity about it – took place three years after the events for which Mr Mabior was on trial.

Many individual doctors have been saying in private (and to patients) that the Swiss statement is essentially correct, and so an individual who has been counselled in this way could state that they honestly believed that he or she was not infectious.

However, a court may also have to separate the science of the Swiss statement (i.e. deciding whether exposure to HIV could have taken place) from other sections of the Swiss guidance. This is because the Swiss recommend disclosure before any unprotected sex, and say that the decision whether or not to abandon condoms should be made by the HIV-negative partner.(ix) This reflects ethical issues underpinning many HIV-disclosure laws. These laws state that informed consent by the HIV-negative partner to the risk of HIV exposure (however low or insignificant) is a necessary requirement to make the very act of sex itself consensual.

i Quinn CT et al. Viral load and heterosexual transmission of human immunodeficiency virus type 1. New England Journal of Medicine 342 (12): 921-929, 2000.

Varghese B et al. Reducing the risk of sexual HIV transmission: Quantifying the per-act risk for HIV on the basis of choice of partner, sex act, and condom Use. Sexually Transmitted Diseases. 29(1):38-43, 2002.

Fisher M et al. UK Guideline for the use of post-exposureprophylaxis for HIV following sexual exposure. International Journal of STD & AIDS 17: 81–92, 2006.

Powers KA et al. Rethinking the heterosexual infectivity of HIV-1: a systematic review and meta-analysis. Lancet Infect Dis 8: 553–63, 2008.

v Wolf LE and Vezina R. Crime and punishment: is there a role for criminal law in HIV prevention policy? 25 Whittier L. Rev. 821, 2004.

“AA” [January 2005 judgment of Supreme Court of the Netherlands] Criminal Section no. 2659/03 IV/SB. Quoted in Brands R. Decision of the Dutch Supreme Court and its implications. Criminalisation of HIV transmission Community Workshop, Eighth International Congress on Drug Therapy in HIV Infection, Glasgow, November 2006. www.eatg.org/Projects/Criminalisation-of-HIV-transmission/Downloads-slid...

vii Vernazza P et al. Les personnes séropositives ne souffrant d’aucune autre MST et suivant un traitment antirétroviral efficace ne transmettent pas le VIH par voie sexuelle. Bulletin des médecins suisses 89 (5), 2008.

Garnett GP and Gazzard B. Risk of HIV transmission in discordant couples. Lancet 372: 270-71, 2008.

Swiss Aids Federation. Advice Manual: Doing without condoms during potent ART. January 2008. Available at: www.aidsactioneurope.org/fileadmin/files/About_us/projects/WirksameART__...



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