Would treatment work as prevention for gay men? McCormick, 2007

A 2007 model estimating the effect of antiretroviral therapy (ART)[ix] on HIV-transmission rates among men who have sex with men (MSM)[ix] came to much less optimistic conclusions.¹ This study estimated that antiretroviral therapy (ART), if used in all MSM with CD4 cell counts below 350 cells/mm³, would reduce infections by 26% over one decade – but only if there were no other changes in risk behaviour.

This model took account of some of the paradoxical effects of treatment. Since ART reduces HIV viral loads in both blood and semen, it could reduce infections resulting from people who are on therapy. Conversely, ART might lead to more infections as HIV-positive people on treatment remain alive and sexually active longer. A further complication is whether, and by how much, antiretroviral therapy affects sexual-risk behaviours.

The analysis compared two hypothetical cohorts of MSM: one not on treatment, and one in which all men with CD4 cell counts below 350 cells/mm³ were treated. Each person in the model was assumed to have between 0.8 and 2.7 acts of unprotected anal intercourse per month (assumptions derived from US studies). The per-contact infection risk was stratified according to viral load, with account taken of higher infectiousness in primary infection and late-stage disease.

Untreated individuals were taken to survive a mean of just over twelve years and treated individuals 30.6 years.

In the analysis, a single untreated individual was calculated to cause a total of 1.9 new infections after ten years, 2.5 after 20 years, and a total of 2.5 new infections after 30 years (estimated mean numbers).

The model found that treating an individual would cause only slightly fewer cumulative secondary infections: 1.4 after 10 years, 1.8 after 20 years, and 2.3 after 30 years. After 33 years, the treated individuals actually began to cause more secondary infections than the untreated (due to treatment failure and longer survival); the treated group caused 23% more infections than the untreated group over an entire simulated lifetime.

In a ‘sensitivity analysis’ component of the study, results were found to be affected greatly by changes in sexual behaviour. If risky behaviour doubled in men for whom treatment was available (regardless of whether they actually started), this led to an average of 2.8 new infections per treated person over the first ten years – twice as many as if no change in behaviour was assumed, and 1.5 times more than the untreated men.

One drawback of this model is that the researchers assumed that each act of unprotected anal intercourse (UAI) was with a new partner who was HIV-negative. They say that this was because of the “rapid rate of partner turnover in the population of interest”, but this may overestimate partner change. In many populations, UAI is more common in regular relationships than in casual partnerships.

The authors commented that their results, “although still predicting that treatment would reduce infectiousness over the first ten years, are less optimistic...regarding the likelihood of HIV eradication.” They concluded that: “ART alone will not eradicate the HIV epidemic.”