Just how risky is HIV transmission? Part II
As expected, sexual exposure risks ranged from too low to quantify for oral sex, to 138 per 10 000 for receptive anal sex. Falling in between were insertive anal sex (11 per 10,000), receptive vaginal sex (8 per 10 000), and insertive vaginal sex (4 per 10 000).
These updated estimates for both receptive and insertive anal sex are substantially higher than previous estimates (1.8-fold and 0.7-fold higher, respectively).
Factors that increase transmission include high viral load and STIs whilst ARTs, circumcision and condoms reduce the likelihood of infection. Specifically, having acute HIV infection was about seven times riskier—and late-stage HIV disease was nearly six times riskier—than asymptomatic HIV disease. Having high blood viral load nearly tripled the risk, while STIs that cause genital sores more than doubled the odds of HIV transmission.
Various prevention measures, on the other hand, dramatically lowered the risk of transmission. Most effective was early ART for the HIV-positive partner in a mixed-status couple, which reduced the risk by 96 percent (as shown in the HPTN 052 trial).
Consistent condom use reduced the risk of HIV transmission by 80 percent. Voluntary male circumcision—which has been shown to be an effective strategy in Africa, but less so among gay men in wealthier countries—was associated with a 50 percent decrease in risk for heterosexual men, but a small increase for some men who have sex with men.
It is important to emphasize that a person could become infected from having unprotected sex once or a person could have unprotected sex many times and not become infected, regardless of how low or high the risk per exposure is.
A risk of 1 percent would mean that an average of one infection would occur if 100 HIV-negative people were exposed to HIV through a certain type of sex. It does not mean that a person needs to be exposed 100 times for HIV infection to occur.
The numbers are rough estimates. They are averages and do not represent the risk from all exposures to HIV through a certain type of sex.
Although the risk of HIV transmission from a single exposure may seem low to some people, this risk increases over multiple exposures. In other words, a person who is exposed to HIV more often has a greater overall risk of HIV transmission than someone who is exposed less often.
Taking measures to avoid an exposure in the first place (for example, through the correct use of condoms or other barrier methods,) can help reduce the overall risk of HIV transmission.
(Remixed from betablog.org and www.catie.ca)
