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:
- The one-year risk was highest when anal sex was practiced (ranging from 12%-52%).
- The one-year risk was lowest when the positive partner was on treatment in combination with other strategies (ranging from 0.05%-0.1%).
For MSM couples:
- When no prevention strategies were used at all, the one-year risk was 52% and 10-year risk was 99.9%.
- When consistent condom use only was reported, the one-year risk was 13% and the 10-year risk was 76%.
- When the positive partner was on treatment and consistent condom use was reported (with or without other prevention strategies), the one-year risk was 1% and the 10-year risk was 6% or less.
- Without any treatment, combining PrEP, condoms and practicing insertive anal sex, the one-year risk was 5% and the 10-year risk was 39%.
- Without any treatment, using either PrEP, circumcision or practicing insertive anal sex only, the 10-year risk was greater than 95%.
For heterosexual couples (female positive partner):
- Overall, the one-year risk ranged from 0.01%-12%, while the 10-year risk ranged from 0.1%-71%.
- With treatment, consistent condom use and PrEP, used individually or in combination with other methods, the one-year risk was 2% or less.
- When consistent condom use and treatment were combined, the one-year risk was less than 0.05% and the 10-year risk was 0.5% or lower.
- Other than treatment, no single strategy resulted in a 10-year risk lower than 10%.
- Practicing insertive anal sex almost doubled the risk compared to vaginal sex when only treatment, PrEP or condoms were used.
For heterosexual couples (male positive partner):
- Overall, the one-year risk ranged from 0.05%-20% and the 10-year risk ranged from 0.5%-89%.
- The one-year risk more than tripled when receptive anal sex was practiced, with or without condoms.
- With treatment alone, or in combination with either PrEP or consistent condom use (without receptive anal sex), the one-year risk was 2% or lower.
- With PrEP and consistent condom use (without receptive anal sex), the 10-year risk drops to below 10%. Out of all groups, this was the only group where any strategy without treatment reduced the 10-year risk below 10%.
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 per-act sexual HIV transmission risk estimates may be low, that risk accumulates over time.
- Treatment seemed to have the most protective effect; however, even with treatment, the risk among MSM may be unacceptably high.
- Any anal sex greatly increased HIV risk.
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.