Heterosexual anal intercourse: a neglected risk factor for HIV?

Abstract

Heterosexual anal intercourse confers a much greater risk of HIV transmission than vaginal intercourse, yet its contribution to heterosexual HIV epidemics has been under-researched. In this article we review the current state of knowledge of heterosexual anal intercourse practice worldwide and identify the information required to assess its role in HIV transmission within heterosexual populations, including input measures required to inform mathematical models. We then discuss the evidence relating anal intercourse and HIV with sexual violence.

Fig 1

(a) Prevalence of anal intercourse (AI) reported by female sex workers (FSW) in sub-Saharan Africa. Data taken from a systematic review and meta-analysis of prevalence and frequency of AI among FSW reported in publications from 2002 onwards (details of search strategy available on request). Van Damme et al. reported from four countries of which three are from sub-Saharan Africa (Cote d’Ivoire, Benin, South Africa, the fourth being Thailand), and so it is included here. Errors bars represent 95% confidence intervals (CIs). DRC, Democratic Republic of Congo; NS, not stated; VCT, voluntary counseling and testing attendees. (b) Collection of published studies of prevalence of AI reported by the general population in sub-Saharan Africa. Errors bars represent 95% CIs. ‘Shebeen’ is a beer hall/bar. ‘High-risk’ women refer to women reporting ≥2 partners in the previous 3 months (Opoku) or a collection of different risk types including women with STIs, those with multiple sex partners, and some FSW. ‘Current partner refers to AI at any time with a current partner. GP, general practice; STI, sexually transmitted infection; VCT, voluntary counseling and testing. (c) Collection of published studies of prevalence of AI reported by youth (mean age of sample <25 years) in sub-Saharan Africa. Young adults refer to school-age students, adolescents, and university students. ‘Out-of-school’ refers to youth of school age but not currently attending school. Errors bars represent 95% CIs.

Fig 2

(a) Figure shows women’s cumulative individual-level risk of HIV acquisition by number of exposures to HIV involving unprotected receptive anal intercourse (URAI). Several plots are provided: 1.4% HIV transmission probability per URAI act represents the current estimate for URAI for heterosexual and men who have sex with men couples, based on empirical evidence,^, with 0.2 and 2.5% representing the 95% confidence intervals. The 4.2 and 14% transmission risks per act represent possible increased risk of transmission if the sex act is forced rather than consensual (3- and 10-fold increased risk, respectively), reflecting greater risk of trauma to the mucosa. Calculations based on the relationship: cumulative HIV risk = (1 − β_A)ⁿ where n is number of sex acts (all assumed to be URAI) and β_A is HIV transmission probability per URAI act. (b) Women’s cumulative individual-level risk of HIV acquisition accounting for all sex acts [both unprotected receptive vaginal intercourse (URVI) and URAI]. Plots show different frequencies of URAI within the relationship from 0 to 100% of all sex acts. Risk per act for URVI and URAI are set to 0.08 and 1.4%, respectively [i.e., model represents unforced anal intercourse (AI) or that there is no increased risk of HIV transmission if it is forced]. Horizontal axis is total number of sex acts, but we have included a second axis estimating relationship duration, which assumes 10 sex acts per month, to estimate the rate of transmission within an HIV discordant relationship. Calculations based on the relationship: cumulative HIV risk = (1 − β_A)^nf (1 − β_V)^n(1−f) where n is number of all sex acts, β_A is HIV transmission probability per URAI act, β_V is that for URVI and f is the fraction of all sex acts that are URAI. (c) Schematic of a heterosexual network. Individuals are represented by circles (M, males, F, females, red, HIV-infected, white, uninfected), and lines represent sexual relationships between them. Figure represents a static network (i.e., no partnership formation or dissolution, just a snapshot in time). Thicker lines represent relationships that involve some frequency of AI; thinner lines involve vaginal sex only. Short-term (even just one sex act) partnerships are represented by dotted lines and long-term sexual partnerships by solid lines. Figure illustrates the very different individual-level and population-level HIV risks experienced by women suffering different forms of sexual violence and with different positions in the sexual network.

Fig 3

Box and whisker plot of population-attributable fraction of new HIV infections per year due to AI and primary (acute) HIV infection (PI). Estimations from a mathematical model parameterized using sexual behavior data reported by men and women recruited from townships and sexually transmitted infection (STI) clinics in Cape Town, South Africa. AI, anal intercourse; max, maximum; min, minimum. Plotted dots represent outliers.