In stable gay couples, where one partner is taking HIV treatment and the other is HIV-negative, the risk of HIV transmission is relatively low if condoms are not used following a recent undetectable viral load test result. However, using condoms on a few more occasions but without reference to viral load substantially increases the risk of HIV transmission. These are the findings of a mathematical modelling study, drawing on detailed data on viral loads in Dutch gay men, published online ahead of print in Sexually Transmitted Infections.
The model suggests that during the entire period that a first-line treatment regimen is taken, the risk of HIV transmission would be 1% if condoms are used all the time, 3% if condoms are not used after an undetectable viral load test in the past six months, 17% if condoms are used 30% of the time, and 22% if condoms are never used.
Using condoms is most crucial when patients have not recently (within the past 3 months) had an undetectable viral load measurement.
The model was designed to test the proposition put forward in the Swiss statement: that in long-term, serodiscordant couples, a decision to give up using condoms can be safely made as long as the HIV-positive partner is adhering to HIV treatment and has had an undetectable viral load for at least six months.
However it is important to note that the model does not take into account the increased risk of HIV transmission when one of the partners has a sexually transmitted infection. The Swiss statement emphasised that unprotected sex could only be safe if neither partner had a sexually transmitted infection.
This is not the first modelling study conducted to explore the prevention impact of HIV treatment in Western countries (for example others have been conducted by researchers in Canada and Australia). However it is the first to take into account the factor that deciding not to use condoms might be conditional on the most recent viral load measurement being undetectable.
Researchers from Imperial College London and the HIV Monitoring Foundation in Amsterdam developed a stochastic mathematical stimulation model of viral load trends, frequency of viral load testing and HIV transmission risk, drawing on data from a cohort of HIV-positive people in the Netherlands.
The model looked at transmission risk within a stable relationship between two men of different HIV statuses, where the relationship continued for the duration of the first-line HIV treatment regimen.
The researchers based their assumptions about the relationship between viral load and infectiousness on previous modelling work, adjusted to take into account the increased risk of HIV transmission during anal sex rather than vaginal sex.
They also assumed that each couple has anal sex 100 times a year and that condoms reduce transmission risk by 95% (but not more, because they may break or be used incorrectly).
In order to account for a range of other plausible scenarios (for example, less frequent sex in the relationship or a higher transmission risk), the researchers conducted a series of alternative analyses. The results of these are presented as a combined 95% uncertainty interval: the lower and upper figures represent a range of plausible results.
The estimated risk of transmission during first-line therapy is as follows:
If condoms are never used: 22% (uncertainty interval: 9% - 37%).
The researchers note that the results for using condoms for 30% of sex acts are broadly similar to those of men never using them. Men following the viral load strategy would in fact use condoms on fewer occasions (10% of the time), but their risk of transmitting HIV would be substantially lower.
An additional analysis of alternative viral load strategies showed that only giving up condoms if an undetectable viral load had been recorded in the past three months would further reduce the transmission risk, whereas making the decision based on the most recent viral load test result (even if it was more than six months ago) would increase the risk of transmission.
These results lead the researchers to emphasise the importance of more frequent viral load monitoring and of minimising losses to follow-up. “Without such effort, increases in viral load go undetected, exposing partners to higher risks of transmission.”
They say that their results show that “basing the decision to use condoms on viral load provides substantially more protection to partners than incomplete condom use, provided that the measurement is within the past 3-6 months.”
The researchers conclude: “The implications of this work are that the key message to patients should remain that always using condoms when receiving treatment is the best way to protect partners from the risk of HIV transmission. However, an additional message is that using condoms is most crucial when patients have not recently (within the past 3 months) had an undetectable viral load measurement.”
Hallett TB et al. Estimating the risk of HIV transmission from homosexual men receiving treatment to their HIV-uninfected partners. Sex Transm Infect 2010. doi:10.1136/sti.2010.042622
