Appendix 2: Theoretical risk, population risk & individual risk

Theoretical risk

As HIV can be found all over the body there are instances where in theory there could be a risk of infection but in real life this risk is zero.

The examples in the section Ways HIV is not transmitted are driven more by an over-active imagination than by real-life risks.

Theoretical risks can never be disproved. It is not possible to prove that something can never happen. So some health information refers to some zero risk activities as low risk.

Estimated population risks?

Guidelines often refer to population risks for different activities. For example, giving oral sex to a man with ejaculation is sometimes referred to as a 0.09% risk (9 in 10,000).

Most of the risk statistics come from population studies that use the following equation to find the percentage risk of transmission:

Risk of transmission = (risk that source is HIV positive) x (risk of exposure).

This equation should consider both the transmission risk factors discussed earlier and the percentage of people in the population who have HIV.

For example, risks will be higher in a country where 25% of people are HIV positive compared to a country where the less than 1% have HIV.

Population risk vs individual risk

Population risks are based on many cases where both partners are HIV negative and where the risk is zero.

Using the example above, if out of 10,000 people, nine become positive who only reported oral sex then the population risk (in this population) is 0.09% (9 in 10,000).

But the actual risk for each of those six people at the time they caught HIV was much higher than 0.09%.

Factors including their partner’s HIV status, viral load, genetics, STIs, circumcision etc may mean that the risk when the infection occured may have been 10% or 20% (1 in 10 or 1 in 5).

Someone thinking this is only a 0.06% risk would be wrong. If they had been thinking this might be a 20% risk, they may not have become infected.

This is why population estimates should be used cautiously for personal risk.

HIV transmission is often simplified and these important aspects are missed.

Without this detail, prevention advice only tells half the story.

Too often the detail is left out.

Any risk should be put into context of other factors.

This is especially relevant when you may know very little about your partners health.

What is individual risk of HIV transmission?

Individual risk is very difficult to estimate. For some situations the risk could be much higher. For example, if a negative person is giving a positive person oral sex and the following risk factors were included:

  • The HIV positive person has a very high viral load of 10 million copies/mL
  • The HIV negative person has poor gum health, or recently brushed their teeth, or eaten a packet or crisps that scratched a gum etc
  • The positive person receiving oral sex ejaculates in mouth of his partner

The individual risk here could easily be 90% or 50% or 10% or 1%, it is impossible to say which because this level of detail doesn’t exist in any of the studies available.

It will definitely be much higher than the risk of 0.09% estimated and referenced in many guidelines on HIV transmission.

Risks in daily life

Finally, there is probably a reason to talk in general about attitudes to risk in daily life.

Some people choose risks in their daily life that others would find impossible. Many jobs have much higher risks than others.

Sometimes people do things after having considered the risks. Often the risk is assumed to be so low that ‘it will never happen to me’.

For others, the personal anxiety and worry about all sorts of risks restricts and limits the activities in their lives.

Most people find a balance. Or we like risks in some areas of out lives but not in others.

Flying in a plane or driving a car are all associated with real risks for some people, but the risk is small enough for most people to still travel.

The Canary Islands and San Francisco are both popular holiday destinations. One is an active volcano and the other is on the San Andreas fault line. We overcome these risks because we like holidays in the sun. And we fly there.

Sexual health risks are different in important ways:

  • Condoms can reduce your risk to zero.
  • HIV usually takes many years to progress.
  • HIV treatment dramatically increases life expectancy to one similar to HIV negative people.

Last updated: 1 June 2021.