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A composite transmission model to assess incident HIV-1 cases according to sexual behaviour, partner number and seminal plasma viral load
D. Chan, D. Smith
Prince of Wales Hospital, Albion Street Centre, Sydney, Australia
Background: The HIV-1 RNA load in seminal plasma (SPVL) affects the risk of sexual transmission. We present a composite transmission model to predict incident cases of HIV-1 arising from untreated HIV-1 infected men attending a metropolitan HIV-1 clinic in Sydney.
Methods: From a cohort of 110 homosexually active HIV-1 positive men, 38 untreated subjects completed a survey on sexual practices in the past 3 months. Blood plasma HIV-1 RNA load (BPVL) and SPVL were measured contemporaneously. A Bernoulli model was developed incorporating SPVL, number and serostatus of partners, and number of protected and unprotected episodes of anal intercourse, according to the equation:
P = (1-π)[1-(1-(1-τ)αui)ui(1-(1-τ)αur)ur(1-(1-τ)αpi)pi(1-(1-τ)αpr)pr]
Probability of transmission according to SPVL was determined by sensitivity analysis based on the observed correlation between BPVL and SPVL. We constructed an equation that allowed us to predict SPVL from BPVL.
Results: there was a BPVL threshold below which SPVL was low or undetectable and above which SPVL increased geometrically. The BPVL threshold was similar to that shown in large heterosexual population studies. Seven subjects were predicted to have infected 0.93 HIV-1 negative partners and 13 subjects to have infected 4.28 unknown serostatus partners. Probability estimates were heavily skewed by a small number of subjects with high rates of unprotected sex and multiple partners.
Conclusions: More HIV-1 infections may occur from increased episodes of unprotected anal intercourse with multiple partners of unknown HIV-1 serostatus. Mutual ambivalence about disclosure of HIV-1 serostatus may contribute to decisions about sexual risk taking. The model can be used to counsel individuals or predict epidemics amongst different subpopulations, and to assess behaviour change or the impact of public health interventions.
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