Simulating an epidemic 

Nathan Geffen – a leading voice in the Treatment Action Campaign and founder of GroundUp – is graduating with a PhD for his work on HIV epidemic simulations.

Geffen’s PhD thesis has two components. “The second bit is technical,” he says. “It’s an analysis of several algorithms that can be used for speeding up and improving the quality of simulations of the HIV epidemic and other sexually transmitted infections [STIs].” He explains that mathematical models of STIs often need to account for the diversity, or heterogeneity, of human sexual behaviour and that microsimulations are a type of model well suited to accounting for this heterogeneity. “The other part of my thesis is about how modelling has influenced policy decisions in the past.” As an example, Geffen describes a longstanding debate about when people living with HIV should start ARV treatment. “Should they start as soon as they have been diagnosed as HIV positive or should they start later, when their CD4 cell count drops below a specific level?” In the past the medical guidelines governing this have fluctuated wildly. According to Geffen the most cited model that influenced these decisions was published in 2009 in The Lancet medical journal. It argued for people to start treatment immediately, with some plausability. But it wasn’t until a 2015 clinical study was completed that it was proved that starting ARVs at the time of diagnosis has the best health outcomes for people living with HIV. “This is an example of how modelling can influence policy. It is a fascinating subject because, in contrast to the ridiculous debate over whether HIV causes AIDS, for example, this was a highly rational debate between experts acting on sound evidence but reaching different views. “It attests to both the limits of modelling and its importance in understanding how something is working,” he says. Geffen’s PhD has already resulted in three papers being published, one with a German collaborator that he met online after posting a blog about his work. “I hope to publish further papers too,” he says, “but now that the PhD is done I am going to take a break to focus my attention on GroundUp, for now at least.”