Low Viral Load

Viral load describes the amount of HIV present in an individual’s bloodstream. It suggests how well treatments are working and the current risk of damage to an individual’s immune system.

Low viral load is also associated with reduced HIV transmission risk, particularly among those engaging in heterosexual sex. Despite evidence relating to transmission risk among men who have sex with men remaining ambiguous, growing community awareness about viral load has resulted in many gay men and other men who have sex with men believing low viral load prevents transmission. Consequently, it is vital that discussions about viral load and HIV transmission risk are approached with care and attention to detail.

Undetectable viral load

HIV treatments aim to reduce viral load to the point that it is undetectable in blood (below 40 to 50 copies per millilitre) using current commercial tests. Undetectable viral load does not mean HIV is no longer present but that it is present in such low amounts that CD4 cells and the immune system generally experience little damage.

Evidence of reduced transmission

In 2011, the HPTN 052 randomised controlled trial supported evidence from a number of observational studies that antiretroviral therapy can significantly reduce or prevent sexual transmission of HIV-1 within sero-discordant heterosexual couples. Consequently, the international HIV community has engaged in a rapid rethink of HIV prevention strategies, particularly the use of HIV treatments to lower viral load, thus minimising transmission risk (or ‘treatment as prevention’).

This ‘treatment benefits rethink’ is presenting real challenges concerning ways to communicate the potential of HIV treatments to maximise HIV prevention, without overstating the reduction of transmission risk to individuals. While low viral load no doubt reduces HIV infectivity, the specific impact on transmission risk during individual instances of sex between men remains uncertain – and a matter for heated debate.

A recent authoritative article by Muessig et al in AIDS, suggests that while evidence supporting treatment as prevention for MSM ‘is promising ... major gaps in our knowledge remain.’  The impact of low viral load on unprotected sex differs between acts of vaginal sex and acts of anal sex for reasons including:
  • In the absence of antiretroviral therapies, anal intercourse carries a far higher risk of transmission (1/20 to 1/300) than unprotected vaginal sex and the intersection of low viral load on those variables is not fully understood
  • The pharmacology of antiretroviral agents differs between the urogenital tract (vaginal intercourse) and the gastrointestinal tract (anal intercourse)
  • Viral load tests identify viral load in blood, which may differ from viral load in semen, vaginal fluids or cerebrospinal fluid. Higher levels of HIV have been found in the gastrointestinal tract (anal intercourse) than in blood, with HIV persisting in the rectum even when a patient appears virally suppressed
  • A higher median number of HIV variants may be transmitted in MSM couples than in heterosexual couples, potentially posing greater challenges for drug resistance 
  • Co-infections, such as bacterial sexually transmissible infections (which have recently increased among MSM), amplify the risk of HIV transmission.
Analysts are concerned that individual’s belief in the effectiveness of low viral load as a prevention strategy will increase other risk taking behaviours, particularly sex without condoms. Numerous studies suggest this is likely occurring. Many of those studies have failed to identify a reduction in community viral load resulting in fewer infections. That suggests early treatment may form part of an effective HIV prevention strategy, only if combined with early HIV diagnosis, expanded STI diagnoses and treatment strategies, and behavioural interventions including promotion of condom use. Further research is under way to better understand the impact of treatment on individual’s physiology and the social and cultural environments in which new sexual behavioural trends are evolving.


It is not known how Australian courts will treat emerging evidence that low viral load reduces HIV transmission risk, although arguably they will be cautious in their approach to this emerging and sometimes contradictory area of science. The issue of low viral load was raised a number of times during the appeal of Neal (Victoria 2011). In that case, ‘low viral load’ was used successfully as a defence, but not in relation to whether the accused could/was likely to infect someone; rather whether he ‘believed that he was capable of infecting others with HIV.’ That finding was specific to the charge of ‘attempting to cause another person to be infected with HIV’ under section 19 of Victoria’s Crimes Act, and cannot be extrapolated further.

Recent legal cases from other international jurisdictions are varied. In February 2009, the Swiss Courts quashed the conviction and 18 month sentence of an HIV-positive man who had unprotected sex with two women but did not infect them. In short, the Court accepted that the risk of transmission from a person on successful treatments is negligible (validating efforts of the authors of the contentious ‘Swiss Statement’).

Major developments in Canada suggest other legal systems will be more cautious. In October 2012, the Canadian Supreme Court decisions in R. v. Mabior and R. v. D.C. considered the requirement for individuals to disclose their HIV-positive status prior to sex. The Court found that a ‘significant risk’ of HIV transmission triggers the legal duty to disclose, and that a significant risk is present unless a person has low viral load and use a condom.

Following two years of review, Norway’s Commission on HIV and the Law recently found that undetectable viral load is not a valid defence to a person with HIV having unprotected sex whether or not transmission results, although it may be a consideration in sentencing (see Norwegian Commission on HIV and the Law report).

1 A recent publication by ACON that aims to increase gay men’s knowledge about degrees of risk when having sex, includes the impact of low viral load in its ‘risk calculator’ (see ‘Know the Risk’).

2 Muessig K, Smith M, Powers K, Lo Y, Grulich A, Phillips A, Cohen M. Does ART prevent HIV transmission among MSM? AIDS 2012, 26.

3Vernazza 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), (English translation, including translator’s affidavit, available at: http://tinyurl.com/cpyt5n (Last accessed 1 November 2012)), 2008.


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