O1-S07.01 The relative contribution of anal intercourse and primary infection to mature heterosexual HIV epidemics
Authors: Boily M C 
Published: July 2011

BackgoundCurrent epidemiological evidence suggests that receptive anal intercourse (RAI) considerably increases the risk of HIV infection per sex act (RRRAI) compared to vaginal intercourse (VI). RAI may increase HIV risk to a similar extent or more than primary HIV infection (PI)(ie, recent infection) increases infectivity compared to asymptomatic infection (RRPI). Considerable attention has been placed to understand the role of PI to HIV epidemics. However, the potential role of RAI to heterosexual HIV epidemics has never been assessed even if it seems to be a relatively common practice in many settings. We aim to compare the fraction of HIV infections due to AI and PI in a generalised heterosexual epidemic.MethodsA deterministic model of heterosexual HIV transmission during to VI, RAI and insertive AI, incorporating three HIV infectiousness stages was developed (Abstract O1-S07.01 Table 1). Behaviour and HIV prevalence data from Kalishman'set al(2009) study in South Africa was used to define plausible ranges of parameter values (Abstract O1-S07.01 table 1). As it is unknown, the degree of mixing during VI between those who engage and do not engage in AI (non-AI) was varied. 20 parameter sets that best fitted HIV prevalence data by AI/non-AI were identified following exploration of 1000 parameter sets selected by uniformly sampling the plausible parameter ranges.Abstract O1-S07.01 Table 1Main assumptions and results summaryMain parameters assumptionsRanges of values in the 20 best fits*Probability of HIV transmission per VI during asymptomatic phase woman to man / man to woman0.0013–0.0019/0.0013–0.0019Duration / increase in infectivity (RRPI) (relative to asymptomatic phase) in each HIV stagesPrimary/high =6 months/9.2-foldAsymptomatic/low =8 years/onefoldLate/medium =1 year/7.3-foldNumber of sex acts per partner per year30Average number of partners per year2Overall fraction using condoms during AI or VI (Condom efficacy)50% (0.90)a = Degree of mixing between AI and non-AI (a= 0 AI with AI and exclusively VI with exclusively VI, a= 1: proportionate mixing)0.16–0.71RRRAI= % Increase in HIV risk per RAI (compared to VI)7.9 to 19.5-foldRRIAI= % Increase in HIV risk per insertive AI (compared to VI)TwofoldFraction of the population who practice AI11.5%–13.3%Fraction of the population who do not practice AI (non-AI or exclusive VI)86.7–88.5%Fraction of all sex acts of AI group which are AI women / men25.3%–52.7%/21.8%–51.8%Overall fraction of all sex acts which are AI (%AI)2.9–6.7%Annual rate of AI who stop AI and become exclusively VI (numbers who leave AI = no who enter AI from VI, in proportion to their respective HIV prevalence)0.068–0.352ResultsHIV prevalence simulated: Among AI / among non AI (exclusively VI)19.0%–24.8%/6.8%–11.5% Overall population8.5%–12.9%Cumulative fraction of HIV infections (PAF) annually in mature epidemic due to:Anal intercourse Women/men24.7%–54.4% / 9.4%–22.4% Overall population17.3–40.0%Primary phase of HIV infection (ie, recent infections) Women/men23.7%–26.8%/27.3%–37.0% Overall population25.5%–31.2%Ratio of PAF due to AI: PI Women/men1.04–2.14/0.34–0.61 Overall population0.67–1.30* Out of 1000 combinations explored.ResultsOverall, 17%–40% of annual infections (PAF) may be due to RAI and insertive AI (IAI). The PAF due to AI is 2–2.6 larger for female than male (Abstract O1-S07.01 Table 1). In comparison, the overall PAF due to PI is between 25 and 31%, and more similar between gender (PAF female: male 1.0–1.2). Under our assumptions, the PAF due to AI was always larger (smaller) than the PAF due PI for females (males) (Abstract O1-S07.01 table 1). The PAF due to AI and PI was positively associated with increases in the overall fraction of all sex acts which are AI (%AI), whereas the latter depended on the level of mixing. In order to be able to relax the mixing to make it less assortative, the %AI needed to be reduced to allow more VI between AI and non-AI to occur.ConclusionOur preliminary results suggest that even a small fraction of AI (<10%) in a population may be as important, to overall HIV transmission in generalised epidemics, as the primary phase of infection, especially for women. Our results are based on the likely assumption that RRRAI equal or larger than RRPI. Focusing prevention to reduce AI may be more cost-effective than to test and treat for recent infections.