Healthcare professionals are constantly asked to give precise estimates for the likelihood of infection. Categories like 'high risk' and 'low risk' can be perceived as evasive, unhelpful and frustrating for people in search of precise, numeric answers.
This page therefore lists a number of estimates (or guesstimates) of the likelihood of infection per exposure for specific activities and incidents. The figures are drawn from the most useful cohort studies and meta-analyses (which pool the data from as many studies as possible). Nonetheless, close examination of these studies often reveals methodological problems and limitations in the data. The studies do not all follow the same methodologies, making comparisons between figures difficult.
In this section, risk figures are expressed as per exposure estimates. Per exposure means the risk of infection for each individual incident during which HIV transmission could have occurred; e.g. for each separate act of intercourse. This is a more precise measure of risk than ‘per sexual partner’, which does not take into account the number of times sex happens and the fact that sexual partners may or may not have HIV.
The per-exposure measure of risk may cause activities to seem less risky. You might be surprised at the apparently low figures that constitute 'high risk' activities. Being told that there is, for example, a one in 200 chance of infection could, conceivably, lead someone to think, “Only one in 200. Well, that’s not too bad”.
This figure does not take into account the fact that people do certain things (e.g. have sex) a lot more often than they do other things (e.g. prick themselves with an infected needle). This 'one in 200' figure means that the person would only have to have sex with the source partner 140 times for it to become more likely than not that they will catch HIV.
These figures assume an HIV-positive partner. For a partner of unknown status, the risk is affected by the prevalence of HIV in the relevant community – i.e. the chance that the partner does in fact have HIV.
| Activity | Risk-per-exposure |
|---|---|
| Vaginal sex, female-to-male, no condom | 0.04% (1 in 2380) |
| Vaginal sex, female-to-male, no condom, undetectable viral load | 0% |
| Vaginal sex, male-to-female, no condom | 0.08% (1 in 1234) |
| Vaginal sex, male-to-female, no condom, undetectable viral load | 0% |
| Receptive anal sex, no condom | 1.38% (1 in 72) |
| Receptive anal sex, no condom, undetectable viral load | 0% |
| Insertive anal sex, no condom | 0.11% (1 in 909) |
| Insertive anal sex, no condom, undetectable viral load | 0% |
| Receptive fellatio (giving head), no condom, viral load not known | Estimates range from 0.00% to 0.04% (1 in 2500) |
| Pregnancy and childbirth, no preventative measures | 22.6% (1 in 4) |
| Pregnancy and childbirth, undetectable viral load | 0.14% (1 in 715) |
| Injecting drug use | 0.63% (1 in 158) |
| Needlestick injury with contaminated blood | 0.23% (1 in 435) |
| Blood transfusion with contaminated blood | 92.5% (9 in 10) |
A key factor determining the risk of transmission is the amount of virus in body fluids, which is known as viral load. When a person is living with HIV and is on effective treatment, the amount of HIV in their body fluids falls drastically, to a level that is described as ‘undetectable’ viral load. At this stage, HIV cannot be passed on sexually.
On the other hand, a very high viral load increases the risk of HIV transmission. Each log10 increase in viral load is estimated to increase transmission risk threefold (relative risk 2.89). Very recent (acute) HIV infection, a time when viral load is exceptionally high, increases the risk sevenfold (relative risk 7.25). Similarly, advanced HIV disease increases the risk sixfold (relative risk 5.81).
Another factor that would increase the risk of sexual transmission is the HIV-negative person having an STI (genital ulcer disease). This is thought to more than double the chances of acquiring HIV (relative risk 2.65).
The risk can be lowered by prevention measures. When looking at the table below, it is worth bearing in mind that the available figures are not always directly comparable. Some figures are from carefully managed clinical trials, whereas others are from real-world conditions. Some facts can be biologically verified (such as an undetectable viral load) whereas others rely on how people report their behaviour to researchers (such as condom use). For example, if people who occasionally use condoms tell researchers that they always use them, the results for ‘consistent condom users’ included below will include data from people who are actually inconsistent users. These individuals are more likely to acquire HIV and condom effectiveness will be underestimated.
| Prevention measure | Reduction in risk |
|---|---|
| HIV-positive person taking HIV treatment | 96% |
| HIV-positive person taking HIV treatment and having an undetectable viral load | 100% |
| HIV-negative person taking pre-exposure prophylaxis (PrEP) – studies done in high-income countries | 87% |
| HIV-negative person taking pre-exposure prophylaxis (PrEP) – studies done in low and middle-income countries | 46% |
| Condom use during vaginal sex | 71% |
| Condom use during anal sex | 70% |
| Circumcision in HIV-negative heterosexual men | 50% |
| Circumcision in HIV-negative men who have sex with men | 23% |
Baggaley RF et al. Risk of HIV-1 transmission for parenteral exposure and blood transfusion. AIDS, 20: 805-812, 2006.
Baggaley RF et al. Systematic review of orogenital HIV-1 transmission probabilities. International Journal of Epidemiology, 37: 1255-65, 2008. You can read more about this study in our news report.
Boily MC et al. Heterosexual risk of HIV-1 infection per sexual act: systematic review and meta-analysis of observational studies. Lancet Infectious Diseases, 9: 118-129, 2009. You can read more about this study in our news report.
Chou R et al. Preexposure Prophylaxis for the Prevention of HIV Infection: Evidence Report and Systematic Review for the US Preventive Services Task Force. JAMA, 321: 2214-2230, 2019.
Cohen M et al. Antiretroviral treatment to prevent the sexual transmission of HIV-1: results from the HPTN 052 multinational randomized controlled ART. Sixth International AIDS Society Conference on HIV Pathogenesis, Treatment and Prevention, Rome, abstract MOAX0102, 2011. You can read more about this study in our news report.
Giannou FK et al. Condom effectiveness in reducing heterosexual HIV transmission: a systematic review and meta-analysis of studies on HIV serodiscordant couples. Expert Review of Pharmacoeconomics & Outcomes Research, 16:489-99, 2016.
Jin F et al. Unprotected anal intercourse, risk reduction behaviours, and subsequent HIV infection in a cohort of homosexual men. AIDS 23: 243-52, 2009. You can read more about this study in our news report.
Patel P et al. Estimating per-act HIV transmission risk: a systematic review. AIDS, 28: 1509-1518, 2014.
Peters H et al. UK Mother-to-Child HIV Transmission Rates Continue to Decline: 2012–2014. Clinical Infectious Diseases, 64: 527-528, 2017.
Rodger A et al. Sexual activity without condoms and risk of HIV transmission in serodifferent couples when the HIV-positive partner is using suppressive antiretroviral therapy. JAMA, 316: 171-181, 2016. You can read more about this study in our news report.
Rodger AJ et al. Risk of HIV transmission through condomless sex in serodifferent gay couples with the HIV-positive partner taking suppressive antiretroviral therapy (PARTNER): final results of a multicentre, prospective, observational study. The Lancet, 393: 2428-2438, 2019. You can read more about this study in our news report.
Smith DK et al. Condom effectiveness for HIV prevention by consistency of use among men who have sex with men (MSM) in the US. Journal of Acquired Immune Deficiency Syndromes, 68: 337-44, 2015. You can read more about this study in our news report.
Yuan T et al. Circumcision to prevent HIV and other sexually transmitted infections in men who have sex with men: a systematic review and meta-analysis of global data. Lancet Global Health, 7: e436-47, 2019. You can read more about this study in our news report.
