Oral sex risk very low, but not zero, concludes systematic review

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The risk of HIV transmission during oral sex is very low, but not zero, conclude researchers from Imperial College and the London School of Hygiene and Tropical Medicine in the December 2008 issue of the International Journal of Epidemiology. The researchers attempted to identify all the relevant observational studies on the topic, but found that, given the lack of data, it would be inappropriate to make summary estimates for the transmission risk through oral sex.

The authors conducted a systematic review (an analysis of all the medical research on a particular subject that meets predefined requirements). Cohort studies and other observational studies were included, while case reports and reviews were excluded.

The studies reviewed include data from heterosexual, lesbian and gay couples, covering both fellatio and cunnilingus.


oral sex

Kissing, licking or sucking another person's genitals, i.e. fellatio, cunnilingus, a blow job, giving head.


Refers to the mouth, for example a medicine taken by mouth.


Receptive anal intercourse refers to the act of being penetrated during anal intercourse. The receptive partner is the ‘bottom’.

risky behaviour

In HIV, refers to any behaviour or action that increases an individual’s probability of acquiring or transmitting HIV, such as having unprotected sex, having multiple partners or sharing drug injection equipment.

observational study

A study design in which patients receive routine clinical care and researchers record the outcome. Observational studies can provide useful information but are considered less reliable than experimental studies such as randomised controlled trials. Some examples of observational studies are cohort studies and case-control studies.

Only ten studies were judged to be relevant enough to include in the review. All were from Europe or North America, and only three collected their data after the widespread introduction of combination therapy.

Methodological difficulties

The researchers comment on a number of challenges that researchers have encountered in researching this topic.

  • Very few people report oral sex as their sole risk behaviour.
  • If a person practices both unprotected oral sex and unprotected anal or vaginal sex, and acquires HIV, their infection is normally automatically attributed to the higher risk behaviour.
  • Data on individuals’ self-reported sexual behaviour is hard to collect accurately, especially when subjects may prefer to give more socially acceptable answers (i.e. not revealing unprotected anal or vaginal sex).
  • Studies have frequently grouped all oral sex practices together, without distinguishing whether it was receptive or insertive, whether ejaculation occurred in the mouth, etc.
  • Studies of serodiscordant couples (where one person is HIV-positive and the other is not) are likely to include people with well-controlled viral load, meaning that they are much less infectious than during primary infection stage.
  • Studies that do identify a risk from oral sex are more likely to be published and reported than those which do not, because of the interest and comparative novelty of such a finding.

Estimates of the per-partner transmission risk

Five of the studies provided estimates of the risk of multiple oral sex acts during the life of a sero-discordant relationship.

Three of these studies gave that estimate as zero – no transmissions were reported.

The fourth study provided a figure of 1% for receptive fellatio.

The fifth study, from Sweden, provided a much higher estimate of 20%. However, the sample was very small (ten couples reporting oral sex as their only risk factor), and the review authors comment that the high estimate may be due to under-reporting of higher risk activity, or simply due to chance. Moreover, this is the only reviewed study which identified any HIV transmission among heterosexuals that could be attributed to oral sex.

Estimates of the per-partner incidence, per 100 person years

Three of the studies cited in the last section also reported estimates that calculated the transmission risk of multiple oral sex acts, but with the duration of the relationship stated. In each case, the estimate was zero.

Estimates of the per-study-participant transmission risk

Three further studies followed HIV-negative people who reported unprotected oral sex as their sole risk factor. However, the authors note that these studies have additional methodological limitations: the number of sexual partners and their HIV status is not known. This implies that the findings cannot be transferred to other populations where numbers of partners and HIV prevalence are different.

Two American studies gave estimates of 0% and 0.4% respectively, and the more recent Canadian Omega cohort provided a figure of 0.5%. Each study was conducted with gay and bisexual men.

Estimates of the per-act transmission risk

Three studies attempted to calculate the risk of HIV transmission during a single act of oral sex.

Two studies both provided zero estimates – no transmissions were reported.

The third study is Vittinghoff’s often cited paper which used data from American gay or bisexual men who reported multiple risk behaviours. Mathematical models were employed to estimate the risk of different sexual acts, and unprotected receptive oral sex with ejaculation was calculated to have a 0.04% risk of HIV transmission. However, the review authors note that this estimate is based on sex with both infected and uninfected men – if the researchers had been able to exclude sex with HIV-negative partners, the figure would have been higher.


The authors note the paucity of data to inform this review. Reliable estimates would be important for prevention workers and clinicians advising people on the relative risks. Moreover, because of the low risk of transmission, "large and expensive studies" would be required to provide more precise estimates.

They also comment: "The fact that infected study participants with solely this exposure have remained difficult to identify may suggest that indeed the contribution of orogenital intercourse to HIV incidence remains low."

Nonetheless, they do recommend that "individuals should protect themselves using condoms or dental dams to minimise this small risk."


Baggaley RF et al. Systematic review of orogenital HIV-1 transmission probabilities. International Journal of Epidemiology 37 : 1255-65, 2008.