Assessing HIV Transmission Risk
U=U
Where the source HIV status is unknown, the risk of HIV transmission through a single exposure is determined by:
Robust evidence has proven that HIV is untransmissible by sexual activity when the source is on treatment and has an undetectable viral load,33-36 now referred to in global health promotion programs as ‘undetectable = untransmissible’ or U=U.37 However, source information is often unavailable in PEP decision-making. If doubt exists, PEP should be initiated as outlined in Table 2.
- The nature of the exposure with its estimated risk/exposure (Table 1)
- The risk that the source has HIV with a detectable viral load
- Any co-factors associated with the source and the exposed individual
Risk of HIV transmission
=
risk per exposure
X
risk of source having HIV with detectable viral load
What is the HIV transmission risk/exposure?
Table 1 outlines the estimated risks of HIV transmission per exposure to a source with HIV. Whilst these risk estimates are important at a population health level, they do not adequately estimate an individual’s risk after a single exposure. HIV transmission may be increased by numerous factors, including viral load of the source, sexually transmitted infections (STI), breaches in mucosal barriers and circumcision status (see co-factors related to HIV transmission below). In addition, there is considerable genetic heterogeneity between individuals that affects HIV infectiousness and susceptibility.
All sexual risk estimations are for condomless sexual contact. It is assumed that a similar risk is incurred when a condom fails.
Significant exposures are those where body fluids that potentially contain HIV come intocontact with mucosal surfaces or non-intact skin. Body fluids that are infectious, potentiallyinfectious and non-infectious are included in Appendix B.51
| Source HIV status unknown | Source known to have HIV |
|---|---|
| Receptive anal intercourse (RAI) - ejaculation - withdrawal | 1/70 1/155 |
| Shared needles and other injecting equipment | 1/125 |
| Insertive anal intercourse (IAI) - uncircumcised - circumcised | 1/160 1/900 |
| Receptive vaginal intercourse (RVI) | 1/1250 |
| Insertive vaginal intercourse (IVI) | 1/2500 |
| Receptive or insertive oral intercourse | Unable to estimate risk - extremely low |
| Needlestick injury (NSI) or other sharps exposure# | 1/440 |
| Mucous membrane and non-intact skin exposuret | < 1/1000 |
Legend
*These estimates are based on prospective studies, not cross-sectional data or figures derived from modelling. Estimates do not take into account the source viral load. If viral load is undetectable, there is no risk. #Worldwide, there have been no reported cases of HIV acquisition from a discarded needle in a public place.47 Very rare transmission of HBV and HCV have occurred in this situation, so these infections need to be considered. †Human bites and semen splash to the eye are extremely low risk.
Factors modifying HIV Transmission
Many factors modify the risk of HIV transmission and should be considered in the risk assessment.
Viral load (VL):
Other factors that increase the risk of HIV transmission:
- Sexually transmitted infections (STI) in the source or exposed individual, especially genital ulcer disease and symptomatic gonococcal infections
- Source ejaculation during receptive anal, vaginal or oral intercourse
- Breach in genital mucosal integrity (e.g. trauma, genital piercing or genital tract infection)
- Breach in oral mucosal integrity when performing oral sex
- Penetrating, percutaneous injuries with a hollow bore needle, direct intravenous or intra-arterial injection with a needle or syringe containing HIV-infected blood
- The uncircumcised status of the insertive HIV-negative partner practising insertive anal intercourse (IAI) or insertive vaginal intercourse (IVI).
What is the HIV status of the source individual?
Provision of PEP should not be delayed while establishing the source status.
In the setting of non-occupational exposure, the HIV status of the source is often unknown. Previous guidelines have recommended that active attempts should be made to contact the source to request an urgent HIV test, however, this is often impractical and rarely occurs.
Therefore:
- If the source cannot be contacted, seroprevalence data (see below) will assist in determining the need for PEP.
- If the source is contactable and:
- discloses they have HIV, seek consent to determine recent viral load results and antiretroviral treatment history.
- is known to be taking HIV PrEP (pre-exposure prophylaxis), PEP is generally not required. Decisions to prescribe PEP should still be considered on a case-by-case basis due to the potential for PrEP non-adherence of the source.
- chooses not to disclose their HIV status or have an HIV test, it should be assumed (for the purposes of PEP prescription) that they have HIV.
HIV seroprevalence
In Australian populations
Overall, in Australia seroprevalence of HIV is very low at 0.14%, with the highest prevalence among gay, bisexual and other men who have sex with men (MSM) (9.2%)49 The proportion of people with undiagnosed HIV in Australia is greatest in those born in Southeast Asia and Latin America. In people who inject drugs (PWID), seroprevalence is 1.5%, although this figure may be significantly higher than this in those who are also MSM3,49
In overseas populations
HIV seroprevalence overseas varies widely. A high prevalence country (HPC) is defined as having an HIV prevalence of >1% in the general population. However, HIV varies greatly within countries and sub-populations, such as sex industry workers and people who inject drugs, often have higher HIV seroprevalence than the general population. For those exposed overseas, go to https://aidsinfo.unaids.org/ for the most recent seroprevalence estimates.50
What is the HIV status of the exposed individual?
Initiation of PEP should not be delayed while determining the HIV status of the exposed individual.
All candidates for PEP require baseline HIV testing (4th-generation Ag/Ab tests). The results should be followed up on as soon as possible, preferably within 24 hours of the specimen being collected. If this result is positive, refer urgently to an HIV specialist.
Acknowledgement of Country