June 10, 2014
The risk of transmitting HIV among serodiscordant couples may be higher than the zero transmissions seen in the PARTNER study, even when the positive partner is on treatment and condoms are being used consistently, according to a study published in the journal AIDS.
The study estimated the sexual risk of HIV transmission over one- and 10-year periods among MSM (men who have sex with men) and heterosexual serodiscordant couples, using a mathematical model based on current transmission and reduction rates. It is important to note that the model does not predict real-world risk; rather, it points out how risk accumulates over time, using conservative estimates on how much risk is reduced when using different prevention methods.
The following reductions in transmission risk were assumed: 80% from consistent condom use; 54% from circumcision in the negative male partner in a heterosexual couple; 73% from circumcision in the negative partner in an MSM couple; 71% from PrEP (pre-exposure prophylaxis) in heterosexual couples; 44% from PrEP in MSM couples; and 96% from ART (antiretroviral therapy) use by the HIV-positive partner.
For all couples:
For MSM couples:
For heterosexual couples (female positive partner):
For heterosexual couples (male positive partner):
The researchers intended to use this model to comprehensively present short- and long-term risks, using current transmission rates and risk ratios, ultimately highlighting three main points:
While these results may be alarming, the study had many limitations. The researchers did not take into account whether the positive patients on treatment had undetectable viral loads, a key part of reducing transmission risk. There also may have been some social desirability bias when reporting the use of condoms, which could underestimate condom efficacy. The model also assumes that couples are monogamous and had sex six times a month, but as the authors note, "the cumulative probability of HIV transmission changes most when the frequency of sex acts is varied." Lastly, the use of PrEP was not limited to those who had detectable levels in their blood, even though effective PrEP requires adherence.
Moreover, as we've seen in the PARTNER study, there were zero transmissions when the positive partner was on treatment and had an undetectable viral load.
"This model was not designed to predict actual transmission risk for real-world serodiscordant couples over the course of a multiyear relationship. Rather, our intent is to emphasize how risk accumulates over time under various strategies and show the relative differences between strategies," the authors concluded.
Warren Tong is the research editor for TheBody.com and TheBodyPRO.com.
Follow Warren on Twitter: @WarrenAtTheBody.