Studies: heterosexual transmission

Only five prospective studies have so far been published or presented that included couples where the HIV-positive individuals were on treatment. Moreover only three studies measured viral load in the treated partner, and only two included viral load measurements where the limit of detection was below 50 copies/ml. In all studies, all couples were heterosexual.

Castilla and colleagues followed 393 heterosexual couples in Spain between 1991 and 2003 and found that in the 60 couples where the HIV-positive partner was on antiretroviral therapy with a viral load below 50 copies/ml no HIV transmission was observed, compared to a rate of transmission of 8.6% among partners of untreated patients.¹

Melo and colleagues followed 93 heterosexual couples in Brazil over six years, and found that there was no transmission in the 41 couples where the HIV-positive partner was on antiretroviral therapy with an undetectable viral load, compared with six transmissions in the couples where the HIV-positive partner was not on therapy.²

In their meta-analysis, Attia and colleagues calculated that with zero transmissions over a total of 291 person-years of follow-up, the transmission rate was zero. However they note, cautiously, that "the data were also compatible with one transmission per 79 person-years", based on their upper 95% confidence limit of 1.27 per 100 person-years.³

Table 3 summarises HIV transmission rates per 100 person-years according to viral load, where the HIV-positive partner was on treatment.

Source: Adapted from Attia3

In a study in Rakai, Uganda, 205 heterosexual couples were followed for a median of just 1.5 years. Even though only 15 of the 20 HIV-positive partners on treatment had achieved a viral load below 400 copies/ml after six months of treatment, no transmissions were observed in all 20 couples over 1.1 years of follow-up, compared with 34 transmissions where the positive partner was not on treatment.⁴

The largest study to report on the effects of treatment on transmission so far took place in Zambia and Rwanda.⁵ Here, a total of 2993 serodiscordant heterosexual couples were followed-up for a median of 512 days between 2002 and late 2008. However no viral load testing was done. Of 175 transmissions confirmed (by phylogenetic analysis) to have taken place within the relationship, six transmissions occurred in couples where the HIV-positive partner was on treatment, although two of these took place in the first three months following treatment initiation. Excluding the two transmissions early in treatment, the HIV incidence rate on antiretrovirals was calculated to be 0.7 per 100 couple-years, compared to an incidence of 3.4 per 100 couple-years when HIV treatment was not being taken – a fivefold risk reduction.

The final prospective study examining the impact of treatment on transmission took place in Uganda, enrolling 928 HIV-positive individuals, all of whom began antiretrovirals. Among 62 serodiscordant couples, one husband of a female participant became HIV-positive, within the first year of the study. It was estimated that HIV treatment had cut the risk of HIV transmission by 91% over a three-year period.⁶