Viral load and transmission: studies

For most people, antiretroviral drugs (ARVs) can reduce the amount of HIV in the body to very low levels. When the number is below the limit of detection, (usually 40 or 50 copies of HIV’s genetic material (RNA) in each millilitre of blood), viral load is said to be 'undetectable'.

Crucially, this does not mean there is no viable, infectious virus in the body. ‘Undetectable’ means ‘a smaller amount than the test used can detect’, not ‘no virus’.

Research has established1 that most people with ‘undetectable’ viral loads in fact maintain viral loads that can be detected using ultrasensitive tests, with one long-term study finding that the average viral load of people on a boosted protease inhibitor-based regimen, whose viral load was undetectable by a conventional test measuring down to 50 copies/ml, in fact had an average viral load of 3.34 copies/ml. More than three-quarters had a viral load of more than one copy/ml (the ultrasensitive test’s limit of detection).

The amount of virus detectable in other body fluids and tissues may differ considerably from the amount found in blood, and in some cases can be considerably larger.

There may be a difference in the risk of transmission between people who have undetectable viral load but are not taking treatment, and people who have undetectable viral load while on combination therapy. The penetration of antiretrovirals in the genital tract may be important here. It appears that, in untreated people, viral load in blood plasma is less reliably correlated with viral load in the genital tract.

References

  1. Palmer S et al. Low-level viremia persists for at least 7 years in patients on suppressive antiretroviral therapy. Proc Nat Acad Sci 105(10): 3879-3884, 2008
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 It may have been superseded by more recent developments.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.
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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 It may have been superseded by more recent developments.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.