This study aims to move beyond ‘relative age gaps’ to address how much ‘specific age groups’ contribute to HIV risk in men and women in South Africa. To do so, a population-based cohort study from 2004-2015 in rural KwaZulu-Natal, South Africa was analyzed. The study included women 15-49 years and men 15-55 years. This study found non-linear associations between partner age and risk of HIV infection among men and women. 

The general cycle of transmission ensues: men 25–34 years of age transmit recent HIV infection to younger women 15–24 years of age, who then age into a group at greatest risk of transmission to their similar-aged male partners. This cycle of transmission, albeit an oversimplified version of the true dynamics of HIV transmission, is consistent with the age structure of partnerships in this population and provides key insights into potential courses of intervention. Reasons why men (25-34 years) transmit to their younger female partners: they have disproportionately acute and early HIV infections, low ART coverage and high loss-to-follow-up. They are also likely to be in a period of life where multiple partnerships, lower condom use and high coital frequency is common. Although acutely infected young women are also at high risk of transmitting to their HIV-negative male partners, the structure of the sexual network (in which men tend to partner with women their age or younger), results in a delay of high-risk transmission of 5–10 years. Prevention efforts should focus not only on age groups at high risk of HIV acquisition but also for age groups with high potential for onward transmission.