This article first appeared in HIV Update from aidsmap.com.
Two weeks ago HIV update reported on an analysis which showed that even if the risk of HIV transmission during one sexual act is relatively small, the risk can accumulate over time for a couple who have sex regularly. The analysis suggested that effective HIV treatment provides more protection than condoms, but combining both strategies is safer. But when couples (where one person has HIV and the other doesn’t) have sex regularly for several years, the long-term risk may not be negligible.
Another analysis has been produced since then, also focusing on the risk of transmission when a person is taking effective anti-HIV treatment. Its findings are not exactly the same, but it also concludes that it is not possible for scientists to dismiss the transmission risk as zero.
While the two analyses have been done in different ways, both involve making calculations on the basis of previous studies. These are not new studies of new couples.
The most recent analysis was focused on the risks for a heterosexual couple, where one person has HIV and the other does not. The researchers pooled results from several previous studies examining transmission risks. In each study, the researchers had used genetic tests to prove that any new infections really did come from the main partner, not from someone outside the relationship.
They found four cases of HIV transmission in 1672 couples. Three of these infections occurred within the HIV-positive partner’s first six months on treatment and all four within a year. Full viral load suppression may not be achieved until six months after starting treatment.
The researchers calculated the risk of HIV being transmitted during a single sex act. They only considered sex that occurred when the HIV-positive partner had been taking effective treatment for at least six months.
They estimated the maximum risk to be 13 transmissions in every 100,000 sex acts. In other words, a risk of around 1 in 7700 for heterosexual couples. But they stress that these are the maximum figures – the actual figures could be lower, and might be as low as zero.
The uncertainty is because of the small number of couples who took part in previous studies and the limitations of statistics. There’s another, ongoing study which will be able to provide more definitive information when its final results are issued in 2017. And that study will include data on gay men as well as heterosexual couples.