Oral sex and the risk of HIV transmission

The PARTNER 1 and PARTNER 2 studies showed that HIV is not transmitted through condomless vaginal or anal intercourse when a person with HIV is taking antiretroviral treatment and has a fully suppressed viral load (Rodger).

As the risk of transmission through oral sex is estimated to be much lower than for vaginal and anal intercourse in the absence of antiretroviral therapy, it is implausible that the risk of transmission through oral sex is not affected in the same way as other sexual transmission risks when effective treatment suppresses viral load.

When HIV is not fully supressed, the risk of HIV transmission through the mouth is certainly smaller than through vaginal or anal intercourse. If undamaged, the tissues of the mouth and throat are thought to be less susceptible to infection than genital or anal tissues, and an enzyme in saliva also acts to inhibit HIV.

Very few cases of transmission through oral sex have been reported amongst gay men despite the continued practice of oral sex (often with ejaculation into the mouth) by large numbers of men over many years.

Infection via receptive fellatio (‘doing it’) is biologically plausible and may happen occasionally. However, infection via insertive fellatio (‘having it done to you’) is probably exceedingly low risk. There are no reliable reports of HIV being transmitted from the mouth to the genitals.

Cases of transmission via cunnilingus are extremely rare, and the reliability of these reports is questionable.

There are no reported cases of HIV transmission through kissing.

How could HIV infect the mouth and throat?

HIV is not able to infect most cells in the mouth. Only one cell type found in the mouth is vulnerable to HIV infection (Campo).

The tissue of the mouth and oesophagus is also very thick compared with genital tissues, and fluids stay in contact with it for a very short time because swallowing clears the mouth regularly. The mouth is therefore generally regarded as an unlikely route of HIV transmission.

Saliva contains numerous factors that have been found to inhibit HIV and stomach acid is likely to inactivate HIV in the same way as other viruses (Malmud).

Factors influencing risk

Case reports of infections through oral sex suggest that factors which may increase the chance of HIV infection through oral sex include:

• Bleeding gums, cuts or sores in the mouth.
• Inflammation caused by common throat infections, allergies or sexually transmitted infections (STIs) such as gonorrhoea.
• Taking semen into the mouth – all but one of the credible reports of oral transmission note that ejaculation did occur into the mouth of the individual reported to have been infected.
• The presence of genital piercings or lesions.

These conditions could make the tissues more susceptible to infection and/or allow the virus easier access to the bloodstream and immune tissues – similar to the way in which STIs (especially ulcerative STIs) are known to increase the risk of vaginal and anal intercourse.

Brushing the teeth and gums often causes mild abrasions and stimulates bleeding. Safer sex guidelines have sometimes suggested that recent brushing can increase the risk of infection through oral sex.

The risk of HIV transmission through vaginal or anal intercourse is effectively zero if a person with HIV is on treatment and has a fully suppressed viral load. It is logical that if HIV cannot be transmitted through anal or vaginal intercourse when viral load is fully suppressed, the same will apply to oral sex.

Cunnilingus

Cunnilingus is considered very low risk. The very few case reports of HIV transmission are limited to people performing cunnilingus (rather than receiving it). A systematic review included two studies which included cunnilingus in assessments of the risk per oral sex act. In both cases the estimate was zero - no transmissions were reported (Baggaley).

Estimating the risk per exposure

A satisfactory answer to the question, ‘How high is the risk of HIV transmission through oral sex?’ has been notoriously elusive. Collecting reliable data is challenging for several reasons:

• Very few people report oral sex as their sole risk.
• Self-reported data on sexual behaviour are hard to collect accurately, with participants failing to report condomless anal or vaginal sex they have had.
• If a person practises any other form of unprotected intercourse (anal or vaginal) in addition to unprotected oral sex, any resulting HIV infection is usually attributed to the higher risk behaviour.
• Studies have frequently grouped all oral sex practices together, often not distinguishing receptive from insertive roles, whether ejaculation occurred in the mouth, etc.

Many reports of oral transmission are in the form of isolated and anecdotal reports, rather than from observational cohorts (in which people are regularly questioned about their sexual practices and tested for HIV) or other studies with more rigorous follow-up.

Most  cohort studies following men who only practiced oral sex, or serodiscordant couples, have tended to show very low levels of risk, in many cases approaching zero. A few studies have given higher estimates which are difficult to reconcile with the others.

Two authoritative reviews of the evidence of the probability of HIV transmission through oral sex both concluded that, given problems with the available data, it would be inappropriate to provide a precise numerical estimate (Baggaley, Patel). The second review did nonetheless suggested that the figure could be somewhere between 0% and 0.04% per act.

If the per-contact risk of oral transmission is 0.04%, HIV might be passed on in one in 2500 acts of oral sex between serodiscordant people. This 0.04% level of risk (one in 2500 exposures) is approximately 20 times lower than the estimated risk for receptive anal sex, but is only half the risk estimated for receptive vaginal sex with a partner during chronic infection (0.08%).

Many of the studies were carried out before antiretroviral treatment after diagnosis became the norm and so are likely to greatly over-estimate the risk of infection. If viral load is fully suppressed, oral transmission cannot take place. If the majority of sexually active people with HIV are on treatment, the per-contact risk is greatly reduced.