The researchers added that their mathematical model showed that transmission within cohabiting couples occurred largely from men to women.

The findings of the mathematical modelling study indicate that current HIV-prevention efforts, which chiefly target couples where one partner is HIV-positive and the other is not (serodiscordant couples), will be insufficient to bring about major reductions in HIV incidence in the general population.

‘€œBecause of the large contribution of extra-couple transmission (from outside partnerships) to new HIV infections, interventions should target the larger sexually active population and not just serodiscordant couples’€, said Steve Bellan from the University of Texas, Austin, who led the research.

‘€œPre-couple (prior to relationship), extra-couple, and within-couple transmission are all common, and HIV control policies that address all these routes are needed to stem the HIV epidemic in Africa,’€ he said.

In sub-Saharan Africa, where most new HIV infections occur, defining the most-at-risk groups is crucial to targeting intervention efforts effectively. But the proportion of heterosexual HIV transmissions that occur within couples’€”compared with the proportions that occur in single people or in people in extra-couple relationships’€”has been hotly debated.

University of Cape Town actuary and epidemiologist, Leigh Johnson said the findings of the latest study appeared to be consistent with what Johnson and colleagues had published previously.

The latest findings are also consistent with a recent study by Hiam Chemaitelly of Weill Cornell Medical College in Qatar (published in the journal AIDS), which found that only a fraction of new HIV infections occur within identifiable stable discordant couples in sub-Saharan Africa.

To help clarify HIV risk for African couples, the authors of this new study in The Lancet developed a sophisticated modelling system that, unlike previous models, combines serostatus and relationship data from Demographic and Health Surveys (DHS) with country-specific trends for the prevalence of HIV, and estimates of HIV survival times.

They used the model to distinguish the specific routes by which individuals became infected in 27 201 cohabiting couples from 18 sub-Saharan African countries.

The estimates suggest that between 30 and 65 percent of all new HIV infections in men and between 10 and 47 percent in women within stable partnerships are the result of extra-couple transmission.

Other important findings to emerge were that transmission in couples occurs more from men to women than vice versa, and that women have a period of high infection risk before entering a cohabiting partnership’€”emphasising the continuing need for prevention strategies aimed at young women.

The researchers believe that despite its expense and logistic demands, a test-and-treat strategy that targets all heterosexual routes of transmission could be key to fighting the HIV epidemic. Test-and-treat involves placing individuals on antiretroviral treatment as soon as they are diagnosed, no matter the progression of the disease. ‘€“ Health-e News Service