We now know that starting antiretroviral therapy early, pre-exposure prophylaxis (PrEP) and vaginal microbicides can all have an impact on HIV transmission, Victor de Gruttola told a satellite session at the International AIDS Society conference in Rome on Sunday. But researchers now need to do more than establish efficacy, he said.
Studies need to identify the mechanisms by which interventions do and do not work in different communities.
They need to get to understand the characteristics of sexual networks, sexual behaviour and local epidemiology that influence their effectiveness. And they need to compare the impact of providing a stand-alone intervention with that of combined packages of interventions.
Other speakers at the satellite, which had been organised by AVAC and the European AIDS Treatment Group, emphasised the importance of implementation research – identifying barriers to the implementation of prevention interventions and developing strategies to overcome them.
Both Victor de Gruttola from the Harvard School of Public Health and Timothy Hallett from Imperial College London suggested there is no single best intervention – or even best package of interventions, but that this will depend on the characteristics of different communities and epidemics.
For different settings, researchers need to identify the combination of prevention interventions which could keep the spread of HIV under control. They also need to establish the breadth of programme coverage that is required.
Timothy Hallett presented some results from a basic mathematical model which aimed to identify the impact and cost of providing antiretroviral therapy to 80% of people at a number of different CD4 counts, PrEP to varying proportions of young people, PrEP to most people of all ages, or a combination thereof.
For each level of spending, Hallett identified the programme that would have the greatest impact – at the lowest levels of spending identified, this would be antiretroviral therapy alone. Should there be budget available to fund more than making therapy available for all with diagnosed HIV, policy makers should then provide PrEP for young people, and then for people of all ages.
But the model’s results change if baseline assumptions shift. If the costs of PrEP are actually lower than Hallett estimated (because drug prices come down), or if it turns out to be more expensive to get people diagnosed early and on to treatment (because testing promotion has less impact than anticipated or because new health services need to be provided), strategies with a greater reliance on PrEP would start to make more sense.
And the modelling studies need to consider other issues. Interventions – and combinations of interventions – will have different levels of effectiveness in different places, depending on a vast range of local factors which researchers are only beginning to get to grips with.
For example, Victor de Gruttola mentioned assortativity: the tendency for people who have many sexual partners to choose partners with the same characteristic. When this is the case, interventions will have less impact than when there is less assortativity.
Other important local factors are the number of transmissions that are due to people who are themselves recently infected, the proportion of people with HIV who are diagnosed and linked to care and the proportion of HIV-negative people who can be provided with an intervention.
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