Viral load and the risk of transmission

Published: 12 August 2013

This section will examine:

  • The link between viral load as measured in the blood, and the risk of sexual transmission.

  • The impact of high viral load during primary infection on sexual transmission risk
  • The effect of antiretroviral treatment on heterosexual transmission risk.

  • The effect of antiretroviral treatment on sexual transmission risk between men.

  • Factors affecting stable blood plasma undetectable viral loads.

  • The relationship between blood viral load and that in the semen, cervix/vagina and rectum.

  • What experts have said regarding viral load and the risk of HIV transmission on antiretroviral treatment, and what guidance is provided for individuals.

Lowering viral load is also crucial in the prevention of HIV transmission from a mother to her baby, but this is covered in detail in a separate section.

Viral load and the risk of transmission: introduction

Viral load is the term used to describe the amount of HIV circulating in the body, as measured in the blood. For people living with HIV, viral load testing of a blood sample is routinely undertaken in the HIV clinic every three to six months in order to help inform treatment decisions. It is measured in terms of the number of copies of HIV’s genetic material (RNA) per millilitre of blood.

The measurement is a 'snapshot' of how much HIV is found in the blood of an individual at the moment of the blood test. Although this may fluctuate between viral load tests – and HIV may be present at different levels in other parts of the body, including the brain, breast milk, genital fluids, the gut and the mucosal lining of the vagina and the rectum – the viral load test is generally considered to be a broadly satisfactory surrogate marker of HIV levels throughout the body.

Studies published as early as 20001 showed that the risk of sexual HIV transmission correlates with the level of blood plasma viral load, and attention to viral load was already being recognised as a risk-reduction method in a subset of highly treatment-literate HIV-positive gay men in Sydney as early as 2005.2

However, it wasn’t until 2008, following a consensus statement from the Swiss Federal AIDS Commission3 that there was a much greater focus on the impact of viral load on sexual transmission risk.

The ‘Swiss statement’ asserted for the first time that the risk of sexual HIV transmission was not just greatly reduced when blood plasma viral load was below detectable levels (which they defined as less than 40 copies/ml) but that after six months and following specific criteria, the risk was as low as 1 in 100,000, which they noted was within the normal bounds of everyday risk . The specific criteria are that the person with HIV:

  • remains adherent to their antiretroviral therapy;
  • is evaluated regularly by their HIV clinician; and
  • has no other sexually transmitted infections (STIs).

In 2011, a randomised controlled trial known as HPTN 0524 conclusively demonstrated that viral load reductions caused by antiretroviral therapy significantly reduced the risk of sexual transmission. The trial found that the transmission risk was lowered by 96%. The impressive results of HPTN 052 have led to a re-evaluation of HIV prevention and treatment access programmes worldwide.

Michel Sidibé, executive director of UNAIDS, said at the time of the release of the study’s results: “This breakthrough is a serious game changer and will drive the prevention revolution forward. It makes HIV treatment a new priority prevention option.”5

The results have also influenced the most recent guidelines from the World Health Organization, which now recommend earlier treatment to benefit individual and public health.6


  1. Quinn TC et al. Viral load and heterosexual transmission of human immunodeficiency virus type 1. N Engl J Med 342(13): 921-929, 2000
  2. Van de Ven P et al. Undetectable viral load is associated with sexual risk taking in HIV serodiscordant gay couples in Sydney. AIDS 19(2): 179-184, 2005
  3. 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), (English translation, including translator’s affidavit, available at: Date accessed: 21 July 2013), 30 January 2008
  4. Cohen M et al. Antiretroviral treatment to prevent the sexual transmission of HIV-1: results from the HPTN 052 multinational randomized controlled ART. Sixth International AIDS Society Conference on HIV Pathogenesis, Treatment and Prevention, Rome, abstract MOAX0102, 2011
  5. UNAIDS Groundbreaking trial results confirm HIV treatment prevents transmission of HIV. Press Release, 12 May 2011
  6. World Health Organization Consolidated guidelines on the use of antiretroviral drugs for treating and preventing HIV infection. WHO, June 2013
This content was checked for accuracy at the time it was written. It may have been superseded by more recent developments. NAM recommends checking whether this is the most current information when making decisions that may affect your health.
Community Consensus Statement on Access to HIV Treatment and its Use for Prevention

Together, we can make it happen

We can end HIV soon if people have equal access to HIV drugs as treatment and as PrEP, and have free choice over whether to take them.

Launched today, the Community Consensus Statement is a basic set of principles aimed at making sure that happens.

The Community Consensus Statement is a joint initiative of AVAC, EATG, MSMGF, GNP+, HIV i-Base, the International HIV/AIDS Alliance, ITPC and NAM/aidsmap

This content was checked for accuracy at the time it was written. It may have been superseded by more recent developments. NAM recommends checking whether this is the most current information when making decisions that may affect your health.

NAM’s information is intended to support, rather than replace, consultation with a healthcare professional. Talk to your doctor or another member of your healthcare team for advice tailored to your situation.