New research casts doubt on age gap theory of HIV transmission

26 September 2017

Research from South Africa suggests a women’s HIV risk may be driven more by their male partner’s age rather than the age gap between them.

young african couple

New HIV infections among women were highest when they were in sexual relationships with men aged 25 to 34, as opposed to the commonly held theory that older men aged 35 and over presented the biggest HIV risk to women. For men, it was women in the age group 25 to 34 that resulted in the highest HIV incidence rates.

In a study led by the US-based Institute for Disease Modeling, data from around 10,260 women and 7,840 men in KwaZulu-Natal, South Africa was analysed from 2004 and 2015. New HIV infections were documented in 1,788 women and 579 men.

In contrast to the theory that, within sexual partnerships, the age gap between men aged 35 and above and women aged under 25 creates the circumstances that put young women at the greatest risk of acquiring HIV, the study found transmission to be driven more by factors associated with men aged between 25 and 34.

In fact, the heightened risk of transmission was found to affect women in a range of age groups – not just younger women – although young women aged 15 to 24 involved with men in the highest-risk age group of 30 to 34 did have the largest HIV risk. To this ends, the age gap between young women and older men, albeit slightly younger men than had previously been theorised, remained an indicator of transmission.

Adolescent and young women aged between 15 and 24 who reported a recent sexual partnership with a man aged between 25 and 29, followed by women aged between 25 and 49 with a male partner aged between 25 and 29 were the second and third groups found to be most at risk of acquiring HIV. Overall, women with a male partner under the age of 35 had three times greater risk of acquiring HIV than those with partners aged 35 and above.

The study found men aged between 25 and 30 who reported partnerships with women of the same age were the male age group most at risk of acquiring HIV. Overall, men with a female partner aged between 25 and 34 experienced higher risk than those with female partners aged between 15 and 19. This corresponds to the age of peak prevalence in women, which occurs between 5 and 10 years after a woman is most likely to acquire HIV.

This suggests that men aged 25 to 34 are transmitting recent HIV infections to women aged 15 to 24, who, as they age, then pose a greater risk for similar-aged male partners. Although the authors stress that describing the cycle of HIV acquisition in this way gives “an oversimplified version of the true dynamics of HIV transmission”, they argue it provides key insight for prevention interventions within rural South Africa, and possibly other similar, high prevalence settings.

The findings reflect a number of factors associated with younger men in this setting, namely that they are likely to engage in risky sexual behaviour, including multiple sexual partnerships. As they are also less likely to be aware of their HIV status and be on antiretroviral treatment than women who acquire HIV, they are at heightened risk of having a high viral load associated with acute HIV infection, meaning they are more likely to transmit HIV to others.

These results shed new light on the potentially limited role large age gaps have in driving HIV incidence in young women, suggesting that prevention efforts should focus not only on age groups at high risk of HIV acquisition, but also on age groups with high potential for onward transmission.

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Written by Hester Phillips