The risk of an individual acquiring HIV following an exposure is dependent upon the risk that the source is HIV positive where unknown and the risk of infection following a specific exposure from an HIV-positive individual
Risk of HIV transmission = risk that source is HIV-positive x risk of exposure
The probability of HIV transmission depends upon the exposure characteristics, the infectivity of the source and host
Factors increasing the risk of HIV transmission:
- . A high plasma HIV viral load (VL) in the source – with each log10 increase in plasma HIV RNA the per-act risk of transmission in increased 2.9 fold . This may be particularly relevant during primary HIV infection .
- Breaches in the mucosal barrier such as mouth or genital ulcer disease and anal or
vaginal trauma following sexual assault or first intercourse .
- Menstruation or other bleeding – theoretical risk only
- Sexually transmitted infections in HIV positive individuals not on ART or HIV
negative individuals with genital ulcer disease .
- Ejaculation - Among a community cohort of men who have sex with men (MSM) the risk of
HIV acquisition per episode of unprotected receptive anal intercourse with and without
ejaculation was estimated to be 1.43%
- Non-circumcision - circumcision has been shown to significantly reduce HIV acquisition
among heterosexual men in high prevalence countries .
- Discordant HIV viral load in the genital tract - In general, the genital tract viral load is
undetectable when the plasma viral load is undetectable. When this is not the case the viral
load in the genital tract is usually low