There are problems in establishing the comparative degrees of risk of particular activities. This is because:

• They vary with the co–factors described immediately below.
• Different studies have shown some conflicting results in different populations.
• The size of statistical samples has not been enough to do away with a statistical possibility of error of +/– 5%.

However, it is possible to suggest orders of magnitude of risk.

Beyond distinguishing such orders of magnitude, however, any greater precision can rarely form a safe basis for making individual judgements about preventing HIV transmission in particular circumstances.

For example, although it is probably true that unprotected anal sex is more likely to transmit HIV infection than unprotected vaginal sex, any reliance upon protection through switching from one to the other would fail to reduce the risk sufficiently for it to be an acceptable mode of protection. Thus advice to heterosexuals to refrain from anal intercourse in order to be safe is a false form of protection.

Conversely, we know that the correct use of a condom of an appropriate quality will reduce the order of magnitude of risk quite substantially.

The use of the categories `high risk', `medium risk', `low risk' can become confusing, particularly where different agencies or countries classify the same activity at different levels of risk.

However, it is becoming increasingly clear from epidemiological evidence that the activities classified as `high risk' in the earliest years of the epidemic remain the preponderant routes of transmission, whilst those classified in the past as `medium risk' have in fact been discovered to be very low risks.

The very highest-risk activities

• Unprotected receptive anal sex.
• Unprotected receptive vaginal sex.
• Sharing unsterilised injecting equipment.
• Being born to or breastfed by an untreated HIV-positive mother.

High-risk activities

• Unprotected insertive anal or vaginal intercourse.
• Receiving donated blood or organs or injections with unsterilised needles in countries with inadequate screening procedures.

Less risky activities

• Penetrative sex with appropriate barrier (condom or female condom), when used correctly: there is a (not insignificant) residual risk because incorrect use may lead the condom to fail.
• Oral sex receptively.
• Sharing injection equipment if improperly sterilised.
• Occupational risks in invasive surgical and medical contexts.
• Laboratory work with superconcentrates of HIV.
• Being born to or breastfed by an HIV-positive mother receiving antiretroviral treatment.

Very low risk activities

• Sharing penetrative sex toys.
• Sharing razors, toothbrushes, etc.
• Rimming if blood present in stool.

Never mind the facts, just give me the figures

Medical and helping professionals are time and again asked by people afraid they have put themselves at risk of HIV to give precise estimates for the likelihood of infection.

For reasons specified above and below, precise estimates of the risk level of individual activities are hard to come by and vary enormously 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 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 reasons.

However, HIV advisors and recipients of 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 the best estimates (or guesstimates) of the likelihood of infection per exposure for specific activities and incidents.

Per Exposure means the risk of infection for each individual incident HIV transfer could have occurred. 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 problem with this measure of risk is that it makes things look not very risky. What it does not take into account is 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.) Health advisors have been reluctant to give figures like this before, because they felt that if they said to someone who has done the sexually most risky possible thing (had receptive unprotected anal sex with someone recently infected) and tell them they have a one in 33 chance of infection, they will think “only one in 33. Well, that’s worth risking.”

What this one in 33 figure means is that the person would only have to have sex with the source partner 17 times – easily done within, say, a month in a new relationship –for it to become more likely than not that they catch HIV.

However figures can also exaggerate perceived risk, too. The figures below are all on the basis that the source partner is known to be HIV-positive.

Where the serostatus of the source partner is unknown, the prevalence of HIV in the source partner’s risk group has to be taken into account. This makes certain activities very much less risky than they otherwise would have been if the partner was known to be positive. In other cases it makes relatively little difference.

For instance, about one in eight gay men in London has HIV whereas about one in 500 UK adults in general has it (unless they are in another higher-risk group).

So where the source partner’s serostatus is unknown this would mean that the chance of contracting HIV from a single act of unprotected receptive anal sex with a gay man in London is at most one in 33 times eight = one in 264.

However the chance of getting HIV from a similar act of unprotected vaginal sex if you are a woman having sex with a man who is not from a high-prevalence community would be at most 0.00164%, or about one chance in 61,000.

Having said this ‘from a high prevalence community’ is not something that can be easily judged and is susceptible to prejudice.

Table: Estimated HIV risk per exposure for specific activities and events

Activity	Risk per exposure	Notes
Vaginal sex less than five months after infection of source partner	0.82%            (1:123)	Because viral load likely to be higher: see below
Vaginal sex, partner in chronic infection (but not with AIDS)	0.08%          (1:1250)
Vaginal sex, chronic infection, with condom	0.0012%      (1:8333)	Assuming condom failure rate of 15%
Vaginal sex, partner with AIDS and not on treatment	0.45%            (1:222)	From study in Africa: sex with partner 5-15 months before their death
Insertive vaginal sex (overall)	0.06%          (1:1666)	Eight to ten times more risky if partner recently infected
Receptive anal sex less than five months after partner’s infection	3%                   (1:33)
Receptive anal sex = five months after partners infection, with condom	0.45%            (1:222)	Condom failure rate 15%
Receptive anal sex (overall)	0.82%            (1:123)	Estimated range 1:50 to 1:400
Insertive anal sex (overall)	0.06%          (1:1666)	Eight to ten times more risky if partner recently infected
Receptive oral sex	0.04%          (1:2500)	Maximum estimated figure
Sharing injecting equipment	1.4%                (1:71)	Estimated range: 1:48 to 1:150
Needlestick injury	0.3%                 (1:333)	With hypodermic
Mucous membrane exposure (semen or blood in eye, etc)	0.09%          (1:1111)
Blood transfusion	62%                 (6:10)