Antiretrovirals (ARVs) can theoretically be used to prevent HIV transmission in two ways:

Firstly, when taken by people with HIV, they might suppress the virus in the person’s body in a way that makes it harder to transmit to others.

Secondly, when taken by people who do not have HIV, they might prevent the virus from establishing itself in the body.

The first effect may well exist, but it is usually thought unwise to rely on it as a means of HIV prevention. Why not?

For one thing, viral load tests do not measure virus that is present inside cells. These can be transmitted through blood transfusions or through needle-sharing, so an ‘undetectable’ viral load is not going to prevent transmission in this way.

Also, viral load in blood may not match that in semen or vaginal fluids, especially when a sexually transmitted infection or other reproductive tract infection is present. Some ARVs are more effective in suppressing viral load in seminal fluids than others. While some ARVs are concentrated in semen, others appear unable to enter it.

Even if viral load in the blood were an accurate guide, it would only be as good as the last viral load test. Any virus that is transmitted in the context of a rising viral load due to treatment failure would probably be drug-resistant.

There has already been a report of HIV transmission occurring to a steady sexual partner during a treatment interruption. Paradoxically, this does suggest that the treatment was previously a factor preventing HIV transmission (Tubiana).

One context in which further research on this effect is planned is in the provision of ARVs to women who are breastfeeding. World Health Organization (WHO) has developed a protocol for international clinical trials that will seek to find out whether, and to what extent, ARV treatment for mothers can prevent breast milk transmission to their children. This could have major advantages in settings where formula feed is unsafe and unaffordable: in fact, the cost of ARVs for the mother is equivalent to the cost of formula feed for the baby and – it may be argued – could be a better use of limited resources.

The value of ARVs, given to HIV-negative people to prevent HIV being established, is best illustrated in the case of two-dose nevirapine prophylaxis, used to prevent transmission from mothers to babies. Here, the dose taken by the woman at the onset of labour does nothing to suppress her virus. The only way in which it can act is by reaching the baby across the placenta, from where it has been shown to reach levels sufficient to prevent HIV infection. A second dose of nevirapine, given to the baby 72 hours after birth, extends this protection long enough to ensure that any virus to which the baby was exposed during birth is unable to establish itself. This is discussed further in the context of preventing mother-to-child transmission.

The most challenging option that is emerging is to use ARVs to prevent sexual transmission, by giving them for extended periods to HIV-negative people at high risk.