Systematic Review Finds HIV Transmission via Oral Sex Is Rare but Possible

By Liz Highleyman

Over the course of the HIV/AIDS epidemic, the risk of HIV transmission through oral sex has remained controversial. While it is clear that oral sex poses a much lower transmission hazard than unprotected anal or vaginal intercourse, the exact risk is unclear.

As reported in the December 2008 International Journal of Epidemiology, R. Baggale from Imperial College in London and colleagues performed a systematic review to assess the risk of HIV transmission from "orogenital intercourse."

The investigators searched the PubMed database and bibliographies of relevant articles through July 2007. Out of the 56,214 titles searched, 10 potentially appropriate studies were identified; 2 additional studies were identified through bibliographies and 1 through discussion with experts. The studies included data about oral sex on men (fellatio) and women (cunnilingus), and included both heterosexual and homosexual partners.

A total of 10 studies -- all from North America or Europe -- were selected that provided estimates of transmission probabilities per partner (n = 5); incidence per partner (n = 3); per study participant following initially seronegative individuals whose partners were of unknown serostatus (n = 3); and per sex act (n = 3). Given the small number of studies, a meta-analysis was not considered appropriate.

Results

Only 4 of the 10 studies reported non-zero estimates of transmission via oral intercourse:

2 per partner estimates (20% [95% CI 6%-51%] in a study of 10 participants, and 1% [range 0.85%-2.3%] in a model-based estimate);

1 per study participant estimate (0.37% [95% CI 0.10-1.34%]);

1 per act estimate (0.04% [95% CI 0.01%-0.17%]).

For studies with estimated transmission rates of zero, upper bounds of the 95% confidence intervals tended to be relatively large due to small sample sizes:

9.0%, 12.1%, and 2.8% for per partner estimates;

4.7%, 9.6%, and 1.8 per 100 person-years for incidence per partner;

4.4% for per study participant estimate;

0.45% and 0.02% for per-act estimates.

"There are currently insufficient data to estimate precisely the risk from orogenital intercourse exposure," the study authors concluded. "The low risk of transmission evident from identified studies means that more and larger studies would be required to provide sufficient evidence to derive more precise estimates."

One limitation of this review is that few people report engaging only in oral sex, so it is difficult to determine its contribution to an individual's infection risk. Furthermore, just 3 of the 10 studies were done after HAART came into widespread use, and it has been amply demonstrated that full viral load suppression dramatically reduces the risk of HIV transmission.

Given the remaining uncertainty about the risk of oral sex, the authors recommended that "individuals should protect themselves using condoms or dental dams to minimize this small risk."

Mucosal Antibodies

In a related study, reported in the December 24, 2008 advance online issue of AIDS, Swedish researchers conducted a study to determine whether exposure to HIV via oral sex results in HIV-neutralizing activity in saliva.

Saliva samples were collected from 25 HIV seronegative gay/bisexual men with HIV positive male partners, as well as from 22 low-risk HIV negative healthy male control subjects. The presence of neutralizing activity in saliva was tested in a peripheral blood mononuclear cell-based assay using primary HIV isolates. Study participants also completed self-report questionnaires about their sexual behaviors and possible routes of HIV exposure.

Results

21 of the 25 exposed but uninfected individuals reported unprotected receptive oral sex and 3 reported unprotected receptive anal intercourse.

Whole saliva from both exposed uninfected individuals and low-risk control subjects exhibited HIV-neutralizing activity.

However, a significant difference was seen when analyzing the salivary IgA1 fraction.

13 of 25 exposed uninfected individuals showed neutralized HIV, but none of the 22 presumably unexposed control subjects had this capacity.

The neutralizing capacity of the exposed uninfected men persisted through 2 years of follow-up.

In conclusion, the study authors wrote, "Unprotected oral sex evokes a salivary IgA1-mediated HIV-neutralizing response that persists over time during continuous exposure in uninfected male partners of infected men."

These findings may help explain why the risk of HIV transmission through oral sex is so much lower than the risk of anal or vaginal sex.

Department of Medicine, Solna, Unit of Infectious Diseases, Center for Molecular Medicine, Karolinska Institutet, Karolinska University Hospital, Karolinska, Sweden; Gay Men's Health Clinic (Venhälsan), South Hospital, Stockholm, Sweden.

1/13/09

References

RF Baggale, RG White, and MC Boily. Systematic review of orogenital HIV-1 transmission probabilities. International Journal of Epidemiology 37(6): 1255-1265. December 2008. (Abstract).

K Hasselrot, P Saberg, T Hirbod, and others. Oral HIV-exposure elicits mucosal HIV-neutralizing antibodies in uninfected men who have sex with men. AIDS. December 24, 2008 [Epub ahead of print]. (Abstract).