Oral sex has near-zero HIV risk according to new San Francisco study

This article is more than 20 years old. Click here for more recent articles on this topic

The risk of a gay man acquiring HIV from oral sex is extremely low according to a US study published in the November 2002 edition of the journal AIDS.

Investigators recruited 239 gay men seeking anonymous HIV testing in San Francisco between December 1999 and 2001. The men were asked to complete a questionnaire about the type of sex they had had in the previous six months prior to having an HIV test. Although over 10,000 men attended for testing and completed the survey, only 239 reported having oral sex exclusively and were eligible for the study.

The San Francisco researchers were also keen to identify the population-attributable risk percentage of oral sex, as even a very small individual risk of HIV transmission from oral sex for an individual could result in a substantial number of infections in the population as a whole.



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

oral sex

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


Studies aim to give information that will be applicable to a large group of people (e.g. adults with diagnosed HIV in the UK). Because it is impractical to conduct a study with such a large group, only a sub-group (a sample) takes part in a study. This isn’t a problem as long as the characteristics of the sample are similar to those of the wider group (e.g. in terms of age, gender, CD4 count and years since diagnosis).


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

On average, the men in the study had had receptive oral sex with three different men in the past six months (range 0 – 400). The overwhelming majority, 98%, of oral sex was without a condom and 35% of men reported getting semen in their mouths, 70% of whom swallowed.

None of the men in the study tested HIV-positive, meaning that the individual risk of being infected with HIV by oral sex was zero. As the average number of oral sex partners in the past six months was three, the investigators also calculated the population attributable risk percentage for men with one, two and three partners. Although the population attributable risk percentage increased with the number of partners, it remained extremely low, at 0.18% for one partner, 0.25% for two partners and 0.31% for three partners.

“The absence of HIV infection detected in this sample confirms …that orally acquired HIV infection is rare” commented the investigators, noting that the prevalence of HIV amongst gay men reporting unprotected anal sex was 5.1% showing “the striking difference in the risk of HIV between those who report exclusively fellatio and those who report higher-risk sexual behaviors.”

The investigators believe that a particular strength of their study was the questioning of men about their sexual practices before HIV-testing, as men with positive diagnoses “may inaccurately report higher risk exposures for reasons including social desirability and recall.”

The researchers also suggest that improper condom usage could result in more cases of HIV transmission amongst gay men than oral sex. However, they acknowledge that their study sample was relatively small and “therefore, we cannot rule out the possibility that the probability of infection is indeed greater than zero.”

This study dramatically contradicts a much reported investigation, also conducted in San Francisco, which suggested that as many as 8% of all HIV infections amongst gay men were due to oral sex. The conclusions of this study were subsequently undermined, and a recently published Spanish study involving heterosexual couples where one partner was HIV-positive and the other negative, concluded that not a single case of HIV transmission could be attributed to oral sex in a ten year period.

The authors of the new San Francisco study conclude that “it is important that health professionals…have valid information to impart to their sexually active clients ... acquiring HIV through fellatio is significantly less risky than from anal sex, and therefore one’s choice of sexual practices do matter.”


Page-Shafer K et al. Risk of infection attributable to oral sex among men who have sex with men and in the population of men who have sex with men. AIDS 16, 17, 2350 – 2352, 2002.