Anal intercourse
can be practised between two men or between a man and a woman. Despite its
often taboo status impeding research into anal sex between men and women, there
is evidence that a significant minority of heterosexuals have anal sex on a
regular basis in both high-income3 and low-income
settings.4
Unprotected anal
intercourse carries a higher risk of sexual HIV transmission than unprotected
vaginal intercourse. Although either sexual partner can acquire HIV from the
other during unprotected anal intercourse, HIV is more likely to pass from an
HIV-positive insertive partner to his receptive partner than from an
HIV-positive receptive partner to his or her insertive partner.
The most widely cited study of per-act anal-transmission risk was
published in 1999. It found that amongst men in high-income countries,
unprotected anal intercourse with an HIV-positive insertive partner carried an
estimated 0.82% risk of infection for the receptive partner per sexual act, or
a 1-in-122 risk of transmission.5 It estimated
the risk for the insertive partner as 0.06% per sexual act, or a 1-in-1666 risk
of transmission.5
A more recent study amongst men from Sydney,
Australia,6 has been able
to more accurately estimate the per-act risks based on a wider variety of
factors. The data support a recent
meta-analysis of all previous studies of the per-act risk of receptive anal
intercourse to ejaculation for both heterosexuals and sex between men, which
was estimated to be 1.4%.7 Table 5.2 below,
provides an overview of the Australian study's estimates.
Risk of HIV transmission via anal intercourse, per sexual act
|
Per-act probability
|
Insertive partner's risk (circumcised)
|
0.11%
(1 in 909)
|
Insertive partner's risk (uncircumcised)
|
0.62%
(1 in 161)
|
Receptive partner's risk (without ejaculation)
|
0.65%
(1 in 154)
|
Receptive partner's risk (with ejaculation)
|
1.43%
(1 in 70)
|
Source:
Jin
F et al. Per-contact probability of
HIV transmission in homosexual men in Sydney in the era of HAART. AIDS,
published online ahead of print, 2010.
Although the risk for the insertive partner in anal intercourse has
previously been considered to be much lower than the risk for the receptive
partner, this study suggests that for a man who is not circumcised, the risks
of insertive intercourse are broadly similar to the risks of receptive
intercourse without ejaculation.
Few studies have examined the risk of heterosexual unprotected anal sex.
For an HIV-negative woman the per-act risk of anal intercourse is thought to be
around ten times higher than the per-act risk for vaginal intercourse.8 There are no
reliable data regarding the risk for the insertive HIV-negative man of anal
intercourse between a man and a woman, but it is likely that the per-act risk
is similar to sex between men.2
Oral sex (fellatio, cunnilingus, anilingus)
Oral sex can mean fellatio (mouth-penis sex); cunnilingus (mouth-vagina sex); or anilingus (mouth-anus sex). Fellatio is
the only type of oral sex that carries more than a theoretical risk of HIV
transmission, although the receptive partner in fellatio (the person who takes
the partner’s penis into his or her mouth) is still much less likely to acquire
HIV than the receptive partner in anal or vaginal intercourse. The risk for the insertive partner in
fellatio is thought to be virtually nonexistent.
Researchers disagree, however, about the
receptive partner’s exact risk level, with estimates ranging from zero risk
(based on epidemiological studies amongst heterosexuals) to a 1-in-2500 risk of
HIV infection (based on case reports amongst men who have sex with men). The
uncertainty partly reflects the fact that most people who perform fellatio also
engage in other higher-risk sexual activities; HIV transmission in those
situations cannot therefore be attributed to fellatio in particular.
The receptive partner’s HIV risk level in oral sex is generally thought to
be higher if he or she has bleeding gums or has cuts, sores or other abrasions
inside the mouth, including those caused by dental procedures. However, there
is no definitive evidence regarding such factors.
Researchers have identified several likely reasons for the relatively much
lower risk of acquiring HIV from fellatio than from other forms of sex. An enzyme
in saliva inhibits HIV; the low salt concentration of
saliva may also inhibit HIV transmission; and tissues
in the mouth and throat appear to be less susceptible to HIV than are genital
or anal tissues. It is also probable that even if semen is swallowed, digestive
enzymes can destroy HIV in the stomach.