PrEP could prevent up to three million HIV infections in ten years in Africa

Pre-exposure prophylaxis (PrEP) – the use of antiretroviral drugs to prevent infection with HIV – could prevent up to three million new HIV infections over ten years in southern Africa if used consistently, according to mathematical modelling published this month in the online journal PLoS One.

Animal studies are testing the use of combinations of antiretroviral drugs, taken for indefinite periods, in order to prevent HIV infection. Although human studies using tenofovir (Viread) have been troubled by ethical disputes, researchers are increasingly optimistic that combinations of drugs can prevent infection, following a study in macaques which provided evidence of 100% protection in animals that received daily injections of tenofovir and FTC (emtricitabine, Emtriva).

However, incentives to run large trials of PrEP and finance its eventual use are still lacking. Pharmaceutical companies are unlikely to be able to carry out the studies, and will require significant partnership with international donors if the intervention is ever to be tested properly. But understanding of the potential of PrEP is still limited.

Epidemiologists from the University of Pittsburgh School of Medicine and Imperial College, London, set out to model the effects of PrEP under different scenarios in Africa, varying the efficacy of PreP and the effects of sexual disinhibition within the population after its introduction in order to illustrate the range of effects that might be expected.

The model looked at the effect of introducing PrEP in epidemics where adult HIV prevalence has reached 20%.

Theere were three scenarios within the model - an optimistic scenario in which PrEP was assumed to be effective 90% of the time and used by 75% of the sexually active population; a neutral scenario in which PrEP was effective 60% of the time and used by 50% of the sexually active population; and a pessimistic scenario in which PrEP was effective 30% of the time and used by 25% of the sexually active population.

They found a significant public health benefit from PrEP in the optimistic scenario that could potentially cut new HIV infections by 74% in sub-Saharan Africa, if used consistently for ten years. The benefits in the neutral and pessimistic scenarios were significantly lessened - a 24.9% reduction in infections was noted in the neutral scenario and a 3.3% reduction was noted in the pessimistic scenario.

"Our data highlights the enormous potential public health benefit of pre-exposure chemoprophylaxis against HIV, provided the regimen is efficacious and used consistently daily for a number of years," said Professor John Mellors of the University of Pittsburgh School of Medicine.

The researchers also looked at the cost-benefit of distributing PrEP and found that targeting it to individuals who were the most sexually active and thus at the highest risk for HIV infection, produced a significant decline - 28.8% - in infections at a much lower cost than distributing PrEP to the general population.

To address the issue of sexual disinhibition, the researchers assumed a 100% increase in risky sexual behavior and observed that while the beneficial effect of PrEP in the optimistic scenario declined when an individual became sexually disinhibited, there was still a notable reduction of HIV infections in the range of 23.4% to 62.7%.

Substantial unanswered questions about adherence remain however. While African studies have shown high rates of adherence to antiretroviral therapy, this is usually in the context of strong adherence support. African treatment programmes without comprehensive measures in place to promote patient retention have observed high rates of loss to follow-up – approaching 30% in some clinics.