Forecasters agree PrEP/microbicides could cut HIV infections in South Africa

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Several presentations at the Eighteenth Conference on Retroviruses and Opportunistic Infections this week used mathematical modelling to forecast the impact of adopting oral pre-exposure prophlyaxis (PrEP) or a topical microbicide in a high-prevalence country, added to HIV treatment or on its own. Three used South Africa as a model, while one analysed how PrEP might affect a serodiscordant couple.

In this piece we look at the first two models, which look at how PrEP/microbicides might affect HIV incidence and prevalence. The other two models looked at cost-effectiveness (see Modellers examine the cost-effectiveness of PrEP in Africa).

PrEP’s effect on prevalence

Ume Abbas from the Cleveland Clinical Foundation modelled the effect of increased antiretroviral provision with or without additional oral PrEP in South Africa.

She used an optimistic scenario for the expansion of HIV treatment, assuming that antiretrovirals (ARVs) would eventually reach 80% of people with a CD4 count below 200 cells/mm3. She assumed various mortality, dropout and treatment failure rates, and also assumed that, in line with the Partners in Prevention study, that suppressive ARV treatment would cut people’s infectiousness by 92%.



A product (such as a gel or cream) that is being tested in HIV prevention research. It could be applied topically to genital surfaces to prevent or reduce the transmission of HIV during sexual intercourse. Microbicides might also take other forms, including films, suppositories, and slow-releasing sponges or vaginal rings.

drug resistance

A drug-resistant HIV strain is one which is less susceptible to the effects of one or more anti-HIV drugs because of an accumulation of HIV mutations in its genotype. Resistance can be the result of a poor adherence to treatment or of transmission of an already resistant virus.


Refers to the mouth, for example a medicine taken by mouth.


Cost-effectiveness analyses compare the financial cost of providing health interventions with their health benefit in order to assess whether interventions provide value for money. As well as the cost of providing medical care now, analyses may take into account savings on future health spending (because a person’s health has improved) and the economic contribution a healthy person could make to society.


A serodiscordant couple is one in which one partner has HIV and the other has not. Many people dislike this word as it implies disagreement or conflict. Alternative terms include mixed status, magnetic or serodifferent.

This expansion of treatment alone would prevent 696,000 new HIV infections up to the year 2022. However, there would also be 295,000 cases of drug resistance (8.5% of 3.4 million on treatment, a third of it due to transmitted resistance).

She then assumed that PrEP for HIV-negative people was substituted for antiretroviral treatment for people with HIV. This is not a scenario likely to be adopted. If it was, though, it would prevent 200,000 new HIV infections, and there would be a lot less drug resistance (24,500 new cases or 1.1% of 2.2 million on PrEP).

If both ARV treatment and PrEP on this basis were introduced, 839,000 HIV infections would be prevented (20% more than with treatment alone), and there would be 323,000 cases of drug resistance (9.5% of 3.4 million on treatment and PrEP).

This was on the basis of a realistic model of PrEP based on levels achieved in the iPrEx study: the model parameters were that PrEP would reach 50% of those in need of it within five years from now, that it would have 50% efficacy, that people would only take it half the time, and that the average length of time on PrEP would be five years.

With a more optimistic PrEP scenario where it had 70% efficacy, adherence was 80% and people stayed on it for ten years, 1.25 million infections would be averted.

A microbicide’s effect

Valentina Cambiano and a team from University College London modelled the effect of a continued, slow scale-up of HIV treatment with a tenofovir gel microbicide added for women. It was assumed that the gel would have the same efficacy as that seen in CAPRISA 004 and the same adherence characteristics would apply: 40% of the women would use it more than 80% of the time, 40% less than 50% of the time, and the other 20% between 50 and 80% of the time. They assumed that 70% of women in need would have access to tenofovir gel by 2015. Scale-up of ARVs was similar to the Abbas paper, with 65% of people with HIV diagnosed by 2025 and 70% of them on ARVs (45% of all HIV-positive people).

If this scale-up continues but a microbicide is not introduced, the model finds that HIV incidence in the adult population would be 1.13% a year in men and 1.48% in women by 2025. If a microbicide is added, then incidence in men would be 0.85% a year and 0.81% in women. Altogether there would be 32% fewer infections between 2012 and 2025 if a microbicide was used.

Abstracts and webcast

You can view the abstracts from this research on the official conference website:

Abstract 98LB:

Abstract 998:

You can also watch a webcast of the presentations made at this conference session.

Webcast from Advances in PrEP.

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Abbas U et al. Predicting the Impact of ART and PrEP with Overlapping Regimens on HIV Transmission and Drug Resistance in South Africa. Eighteenth Conference on Retroviruses and Opportunistic Infections, Boston. Abstract 98LB. 2011. See the abstract here.

Cambiano V et al. What Impact Would Introduction of a TDF Vaginal Microbicide Have on Incidence of HIV in Women and Men in Southern Africa? Eighteenth Conference on Retroviruses and Opportunistic Infections, Boston. Abstract 998. 2011. See the abstract and download poster here.