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Undetectable viral load and infectiousness
An undetectable HIV viral load is the goal of anti-HIV treatment. This does not mean that a person has been cured of HIV, but rather that the combination of drugs a person is taking has reduced HIV’s ability to reproduce so it can no longer be detected in the blood.
An undetectable viral load in blood does not necessarily mean that a person is not infectious. Although many people with undetectable viral loads in their blood also have an undetectable viral load in their semen and seem less likely to transmit HIV, this is not always the case. Some people with undetectable viral loads in their blood have quite high viral loads in their sexual fluids, which could be high enough to infect somebody else.
Studies have mainly been conducted in men, and these have found that having an untreated STI, particularly gonorrhoea, increases the chances that viral load will be detectable in semen.
In addition, studies have found that a small number of men with high blood viral loads have very high viral loads in their semen and are very infectious.
Also, if a person is resistant to anti-HIV drugs, they can infect other people with drug-resistant HIV. About 10% of new HIV-infections in the UK are with drug-resistant virus. This means that the person newly infected with HIV already has limited treatment options before they have taken a single anti-HIV drug.