Estimated risk per exposure

Medical and helping 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.

However, such precise estimates of the risk level of individual activities are hard to come by and vary widely according to many different criteria:

• Whether the source was known to be HIV-positive.
• If not, the prevalence of HIV in the population or among the specific risk group the person belonged to.
• The stage of infection and viral load of the positive person.
• Whether they were on HIV medication.
• The general state of health of both partners.
• The presence of sexually transmitted infections, including the presence of any ulcers or lesions in either partner.
• The quantity of infectious body fluid transferred.
• The virulence of the particular strain of HIV.
• Whether the recipient has some degree of natural or acquired immunity to HIV…
• …and many other factors.

However, people who give or receive advice have often complained that repeatedly telling someone their activity has been ‘low risk’ is no help unless some degree of quantification is attempted. Therefore, and with all the above caveats in mind, we will here list 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. Moreover, the studies do not all follow the same methodologies, making comparisons between figures difficult.

The sources are as follows: vaginal sex;¹ anal sex;² fellatio;^{3 2} mother-to-child;⁴ other activities.⁵

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’ (because sexual partners may or may not have HIV) or ‘per head of population’ (because people’s risk behaviour varies so much) .

The per-exposure measure of risk may cause activities to seem less risky. Sexually active people may be surprised at the apparently low figures that constitute 'high risk' activities. Telling a person that there is, for example, a one in 200 chance of infection could, conceivably, lead the person 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 100 times for it to become more likely than not that they will catch HIV.

These figures also assume that the 'source partner' is always HIV-positive. 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.

Unless otherwise stated, the sexual acts are always without a condom.

Sources: vaginal sex;¹ anal sex;² fellatio;^{3 2} mother-to-child;⁴ other activities.⁵