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