What is the per-act HIV transmission risk for receptive versus insertive anal sex?
Executive summary
Per-act estimates used by major public-health bodies place the risk of HIV transmission from a single act of condomless anal sex at roughly 1.38% (138 per 10,000) for the receptive partner and about 0.11% (11 per 10,000) for the insertive partner when the partner with HIV is untreated and likely has a detectable viral load [1] [2] [3]. These widely cited central estimates reflect biological differences in exposure and are modifiable — dramatically lowered by condoms, antiretroviral treatment with sustained undetectable viral load, and by an HIV-negative person taking PrEP [1] [4].
1. What the headline numbers mean and where they come from
The commonly quoted “1.38% per act for receptive, 0.11% per act for insertive” comes from systematic reviews and the CDC’s HIV Risk Reduction Tool that consolidated cohort and modeling studies to produce per-10,000-exposure estimates (138 and 11 per 10,000) for anal intercourse with an untreated, HIV-positive partner; those are the basis for public-facing guidance [1] [5] [3]. Different meta-analyses report very similar central estimates for receptive anal intercourse (around 1.4% per act) while insertive estimates are smaller and have been harder to pin down precisely, producing somewhat wider uncertainty in older literature [5] [6].
2. Why receptive risk is higher: the biology behind the gap
Receptive anal partners face higher per-act risk because the rectal lining is thin and easily injured, and semen/pre-seminal fluids can bath a large surface of vulnerable mucosa; rectal tissue and secretions contain target cells and, when inflamed or damaged, allow easier viral entry [7] [8]. Insertive partners are exposed to virus through the urethra, the head of the penis and, if uncircumcised, the foreskin—pathways that carry risk but are biologically less susceptible than rectal mucosa, which helps explain estimates that receptive acts are roughly ten to thirteen times or more riskier than insertive acts in pooled analyses [2] [9].
3. The range, uncertainty and context: not all studies say the same exact number
While the CDC/WHO-style summary uses 138 and 11 per 10,000 as convenient averages, meta-analyses show variation: some pooled estimates put receptive risk around 1.4% per act with wide confidence intervals for per-partner risks, and older or single studies have produced lower or higher insertive estimates (e.g., insertive estimates in some older studies ranged from about 0.06% to 0.62% per act depending on methods and circumcision status) [5] [6] [10]. Reviewers caution that many original studies predate modern antiretroviral treatment, had heterogeneous methods and rely on modeling or partner-prevalence assumptions, so the point estimates should be read as best-available averages, not immutable facts [6] [11].
4. How prevention and clinical factors change those per-act odds
The per-act risks assume an HIV-positive partner with a detectable viral load; an HIV-positive partner who is on effective antiretroviral therapy and durably undetectable has essentially no sexual transmission risk according to current U=U science, and PrEP taken consistently can reduce the receptive partner’s risk by about 99% in some modeling used by the CDC tool [1] [4]. Condom use, treatment of concurrent STIs, avoidance during acute infection, and circumcision for insertive partners also materially reduce per-act transmission probabilities compared with the untreated baselines cited above [1] [10] [12].
5. Practical takeaways and limits of the data
The essential takeaway is that receptive anal sex carries a substantially higher per-act risk than insertive anal sex — commonly summarized as roughly 1.38% versus 0.11% per act in the absence of prevention — but those numbers are conditional: they assume lack of effective treatment or prevention and are averages across diverse studies with methodological limits [1] [2] [6]. Public-health guidance therefore emphasizes layered prevention — testing/treatment to achieve undetectable viral load, PrEP, condoms and STI care — because individual risk on any given act depends heavily on those modifiable factors rather than only on role [4] [1].