Viral load and sexual transmission risk

Viral load is the term used to describe the amount of HIV circulating in the body, and this is usually measured in the blood. Viral load testing of an HIV-positive individual’s 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 mildly from hour to hour – 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.

However, an undetectable viral load result does not necessarily mean that the blood sample is completely free of HIV (although it is possible); nor does it necessarily mean that levels of HIV are the same elsewhere in the body (although, again, it is possible).

In fact, most people with undetectable viral loads will still have some HIV in their blood, and HIV may be present in other parts of the body – including genital fluids and the mucosal lining of the vagina and the rectum. It is currently unknown, however, how frequently these levels change, and whether increased levels of virus in these areas are enough to transmit HIV during sex while levels in the blood remain undetectable. As a consequence there is a great deal of unresolved debate about how infectious somebody with an undetectable viral load in the blood actually is.

In January 2008, a consensus statement from the Swiss Federal AIDS Commission¹ created international controversy by publicly announcing that the risk of sexual HIV transmission was not just greatly reduced when blood viral load was undetectable, but that, under certain circumstances, the risk was as low as 1 in 100,000, which they asserted was within the normal bounds of everyday risk.

They wrote 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 antiretroviral therapy; is evaluated regularly by their HIV clinician; and has no other sexually transmitted infections (STIs); then they are “not sexually infectious, i.e. cannot transmit HIV through sexual contact”.

However many other AIDS organisations, including the World Health Organization (WHO), Joint United Nations Programme in HIV/AIDS (UNAIDS) and the United States Centers for Disease Control and Prevention (CDC) have since stated that there is not yet enough evidence to support this statement, discussed in depth in Guidance for individuals.

However, these organisations do agree that people with an undetectable viral load are less infectious and the WHO, amongst others, is starting to discuss what this might mean on a population level in terms of future public health policy.²