This report provides a synthesis of the current scientific evidence on the risk of transmission of human immunodeficiency virus (HIV) associated with sexual activities, injection and other drug use, and mother-to-child (vertical) transmission. This technical document is intended for use by health authorities and professional organizations to inform the development of policies, programs, and/or guidelines aimed at preventing HIV transmission.
A search was conducted for literature published between January 2001 and May 2012. The search focused on systematic, meta-analytic, and narrative reviews, where they existed. For topics where no reviews existed, primary research studies were included.
Although there are challenges in quantifying risk by sex act, all studies consistently reported that anal intercourse is a higher risk act than vaginal intercourse, which in turn is a higher risk act than oral intercourse. There is also an increased risk associated with receptive intercourse (both vaginal and anal) compared with insertive intercourse.
The risk estimates for the sexual transmission of HIV, per sex act, range widely, from 0.5% to 3.38% (with mid-range estimates of 1.4% to 1.69%) for receptive anal intercourse; 0.06% to 0.16% for insertive anal intercourse; 0.08% to 0.19% for receptive vaginal intercourse (i.e., male-to-female); and approximately 0.05% to 0.1% for insertive vaginal intercourse (i.e., female-to-male). The risk of transmission from unprotected oral intercourse (whether penile-oral or vaginal-oral) is markedly lower than for anal or vaginal intercourse, and findings suggest a low but non-zero transmission probability. The risk of transmission to the receptive partner increases with ejaculation and the presence of oral ulcers and sexually transmitted infections (STIs) in the oropharynx.
The strongest predictor of HIV sexual transmission is plasma viral load. As plasma viral load increases, the risk of transmission also increases. However, much of what is known about viral load and HIV transmission is derived from studies of heterosexual populations. While the nature of the sex acts (i.e., vaginal versus anal intercourse) was not always specified, it is likely that the majority of the sex acts were penile-vaginal. As such, little is known about how viral load affects the risk of transmission through anal intercourse.
The presence of a concomitant STI has also been found to affect HIV transmission. STIs increase susceptibility to HIV by a factor of 2 to 4 and increase transmissibility 2 to 3 times.
Male circumcision decreases the risk of female-to-male sexual transmission of HIV by 50% to 60%. However, there is little epidemiological evidence to suggest that circumcision reduces the risk of transmission to female partners of circumcised men or is effective in the prevention of HIV among men who have sex with men (MSM).
For people who inject drugs, the risk of transmission per injection from a contaminated needle has been estimated to be between 0.7% and 0.8%. However, studies of contact with improperly discarded needles outside of the healthcare setting suggest that such exposures represent a low risk for HIV transmission, likely due to the low viability of the virus outside the body.
Sharing ancillary injecting equipment such as filters or cookers during injection drug use has been shown to increase the risk of transmission, even in the absence of sharing needles and syringes. Other factors that have been shown to increase the risk of HIV transmission for people who inject drugs include injecting in unsafe locations, type of drug used, and frequency of drug injection. While it is likely that viral load is associated with HIV transmission among injection drug users, the number of studies conducted on this topic has been limited.
People using non-injection drugs are also at risk of HIV infection. Drug use can alter sexual behaviours by increasing risk taking. In addition, several drugs have been reported to be independent risk factors for HIV transmission.
In the absence of any preventive intervention, for example, highly active antiretroviral treatment (HAART), mother-to-child transmission (also known as “vertical” transmission) ranges from about 15% to 45% depending on whether breastfeeding alternatives are available. As with other modes of transmission, maternal plasma viral load has been consistently associated with the risk of vertical transmission. Since HAART, which is used to suppress viral replication, was introduced in 1997, the rate of mother-to-child transmission has dropped dramatically in Canada.
Beyond viral load, there are several factors associated with an increased risk of vertical transmission. Concurrent STIs and co-infection with either hepatitis C or active tuberculosis increase the risk of vertical transmission. While mode of delivery was once found to be associated with vertical transmission, since the introduction of HAART, studies indicate that there are probably no additional benefits to elective caesarean section for women with low viral loads.
Obstetric events, including prolonged rupture of membranes and intrapartum use of fetal scalp electrodes or fetal scalp pH sampling, have been found to increase the risk of perinatal transmission of HIV.
Mother-to-child HIV transmission can also occur through breastfeeding. The probability of transmission of HIV through breastfeeding is in the range of 9% to 16%. Co-factors that are associated with risk of transmission from breastfeeding include duration and pattern of breastfeeding, maternal breast health, and high plasma or breast milk viral load.
This review of the scientific literature on HIV transmission was based on over 250 references. Within each route of transmission, estimates of the risk of transmission varied widely, likely due to the role of behavioural and biological co-factors. Viral load (especially in plasma, but also in other relevant body fluids) appears to be an important predictor of transmission, regardless of the route of transmission. However, the evidence indicates that viral load is not the only determinant and that certain co-factors play a role in increasing (e.g., STIs) or decreasing (e.g., circumcision in female to male transmission) the risk of transmission.
This review of the evidence points to the growing and evolving nature of our knowledge of HIV transmission risk and the biological and behavioural co-factors that impact on risk.
Centre for Communicable Diseases and Infection Control
Public Health Agency of Canada
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