Playing the HIV numbers game is less—and more—risky than you think.
Probabilities of HIV transmission per exposure to the virus are usually expressed in percentages or as odds (see chart at the end of this article). For example, the average risk of contracting HIV through sharing a needle one time with an HIV-positive drug user is 0.67 percent, which can also be stated as 1 in 149 or, using the ratios the CDC prefers, 67 out of 10,000 exposures. The risk from giving a blowjob to an HIV-positive man not on treatment is at most 1 in 2,500 (or 0.04 percent per act). The risk of contracting HIV during vaginal penetration, for a woman in the United States, is 1 per 1,250 exposures (or 0.08 percent); for the man in that scenario, it’s 1 per 2,500 exposures (0.04 percent, which is the same as performing fellatio).
As for anal sex, the most risky sex act in terms of HIV transmission, if an HIV-negative top—the insertive partner—and an HIV-positive bottom have unprotected sex, the chances of the top contracting the virus from a single encounter are 1 in 909 (or 0.11 percent) if he’s circumcised and 1 in 161 (or 0.62 percent) if he’s uncircumcised. And if an HIV-negative person bottoms for an HIV-positive top who doesn’t use any protection but does ejaculate inside, the chances of HIV transmission are, on average, less than 2 percent. Specifically, it is 1.43 percent, or 1 out of 70. If the guy pulls out before ejaculation, then the odds are 1 out of 154.
Say what? Is HIV really this hard to transmit, especially in light of the alarming statistics we are bombarded with? Although the CDC estimates that nearly 1.1 million Americans are living with HIV and that the rate of new infections remains stable at about 50,000 per year, there has been a 12 percent increase between 2008 and 2010 among men who have sex with men (MSM)—including a 22 percent jump among young MSM ages 13 to 24.
A report by the Black AIDS Institute states that African-American same-gender-loving men have a 25 percent chance (which is one in four odds) of contracting HIV by the time they’re 25 years old—and a 60 percent chance by the time they’re 40. Other researchers have predicted that half of all gay men in America who are 22 years old today will be HIV positive by the time they’re 50.
So how do we go from the odds being 1 out of 70 that HIV will be transmitted during the most risky sex act to the odds being 1 out of 2 that young gay men in the United States will contract HIV before they’re 50? (And before you even think it: No, the answer is not that everyone with HIV is a ginormous slut who has never heard of safer sex.)
For starters, you have to understand that these probabilities of HIV transmission per single exposure are averages. They are general ballpark figures that do not reflect the many factors that can raise and lower risk.
One such factor is acute infection, the period of six to 12 weeks after contracting the virus. At this time, viral load skyrockets, increasing a person’s infectiousness by as much as 26 times (the same thing as saying “26-fold”). So right there, the per-act risk of receptive vaginal transmission jumps from 1 out of 1,250 exposures to 1 out of 50 exposures, and the risk of receptive anal sex goes from 1 out of 70 to higher than 1 out of 3. It’s also important to realize that during acute infection, the immune system has not yet created the antibodies that lower viral load, at least for a few years. HIV tests that rely on antibodies may give a false negative reading during an acute infection, also known as the “window period.”
The presence of another sexually transmitted infection (STI)—even one without symptoms, such as gonorrhea in the throat or rectum—can raise HIV risk as much as 8 times, in part because STIs increase inflammation and thus the number of white blood cells that HIV targets. Vaginal conditions such as bacterial vaginosis, dryness and menstruation also alter risk.
While this is the end of my story – for now – there’s a lot more in this blog to be explored.