Viral load and the risk of transmission

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 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