The investigation, published in Saturday's issue of the British medical weekly The Lancet, probed the rate of new HIV infections among 2 279 Zambians and 1 782 Rwandans living in towns.
Between 55.1 percent and 92.7 percent of new infections occurred among "serodiscordant" marital or cohabiting couples, it found.
"Serodiscordant" means that, at the start of the study, one partner in a couple had the human immunodeficiency virus (HIV) but the other partner did not.
The finding provides statistical ammunition for those who argue that, in Africa, established relationships are a huge vector for spreading the AIDS virus.
By targeting these couples with voluntary advice about safe sex and encouragement to get an HIV test, health watchdogs could avert between 36 and 60 percent of heterosexually-transmitted HIV infections that would otherwise occur, the paper says.
Lead author is Kristin Dunkle of the Rollins School of Public Health at Emory University in Atlanta, Georgia.
2/3 of Aids sufferers in Africa
Around 33 million people around the world are infected with the human immunodeficiency virus (HIV) that causes Aids, according to the agency UNAIDS. Two-thirds of them are living in sub-Saharan Africa.
Previous research in this field suggests that, for African women, the greatest risk of contracting HIV lies within marriage, but few attempts have been made to determine a man's risk within a marital relationship.
Ignorance about one's HIV status and infections from casual sex outside the couple are the major drivers. – (Sapa/AFP)
Read more:
HIV: how high is your risk?
HIV/Aids Centre
June 2008