HIV Transmission Risk:
A Summary of Evidence
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.
Sexual transmission of HIV
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).
HIV transmission among people who use drugs
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.
Mother-to-child transmission of HIV
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.
To obtain an electronic copy of the report, send your request to:
Centre for Communicable Diseases and Infection Control
Public Health Agency of Canada
100 Eglantine Driveway, Health Canada Building
A.L. 0601A, Tunney's Pasture
Ottawa, ON K1A 0K9
The reproduction is a copy of an official work that is published by the Government of Canada and that the reproduction has
not been produced in affiliation with, or with the endorsement of the Government of Canada.
Source: Public Health Agency Of Canada
Federation of American Societies for Experimental Biologyhttp://www.faseb.org/
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