Q: Is it true that oral sex carries the lowest risks of HIV infection, as opposed to vaginal and/or anal sex? If so, why is this?
A: Yes, oral sex does appear to have a lower risk than vaginal or anal sex. I say “appear” because the only way to fully understand the risk is to study people who only have oral sex and not anything else. Such people would be very hard to find. But we can say pretty confidently that the risk is lower, although not totally absent.
Why you ask?
A couple of factors come into play. First, the oral cavity is not the easiest barrier for bacteria, viruses and other “germs” to cross into the blood stream. This makes total sense because our mouths come in contact with all sorts of things in the environment that we should be protected against. Also, the acidic environment of the stomach is a hostile environment for germs as well. The immune system is also reinforced in this area to attack any germs that breach the barrier. This is an important function of the tonsils (although HIV is tricky and can act as a “Trojan horse” to cause infection).
To strengthen this defense, the lining of the oral cavity, including saliva, contains a variety of compounds that can block HIV infection. Special infection fighting compounds called defensins and mucins can actually trap HIV and keep it from infecting cells. A good colleague of mine, Dr. Habtom Habte, did some very important studies on mucins’s defense against HIV while he was working in South Africa. Other compounds known as CC-chemokines have been shown to protect individuals from infection who have been exposed to HIV through oral contact. So it’s safe to say that the risk is low.
But low doesn’t equal zero!
If either partner has cuts, abrasions or sores on their genitals or in their mouth, this could increase the risk of transmission, particularly if they are bleeding. You should also be concerned that other STD’s can be transmitted by oral sex, such as gonorrhea and syphilis, and both the person performing oral sex as well as the recipient can be infected. For HIV transmission, the person performing either fellatio or cunnilingus is at slightly increased risk of infection than the recipient is. To be perfectly safe, a condom should be used for oral-penile contact. For oral-vaginal contact, a dental dam should be used. This is a thin sheet of latex used by dentists for certain procedures.