Young women in Zimbabwe in sexual relationships with older men have an especially high risk of infection with HIV

Young women in Zimbabwe have an especially high risk of infection with HIV if they have a sexual partner who is ten or more years older, investigators report in the online edition of AIDS. Such inter-generational relationships increased the risk of contracting HIV by approximately 75%.

“We found that age-disparate sexual relationships are associated with increased HIV incidence among young women aged 15-24 years, particularly where partners are ten or more years older,” comment the authors. “Although only a minority of young women reported partners ten or more years older, the proportion was still substantial (21%)…so these inter-generational relationships are likely to be an important contributor to new infections in young women and to the HIV epidemic in this setting.”

The investigators call for the roll-out of interventions, including pre-exposure prophylaxis (PrEP), to reduce the HIV risk for young women in relationships with significantly older men.


person years

In a study “100 person years of follow-up” could mean that information was collected on 100 people for one year, or on 50 people for two years each, or on ten people over ten years. In practice, each person’s duration of follow-up is likely to be different.

adjusted odds ratio (AOR)

Comparing one group with another, expresses differences in the odds of something happening. An odds ratio above 1 means something is more likely to happen in the group of interest; an odds ratio below 1 means it is less likely to happen. Similar to ‘relative risk’. 


In everyday language, a general movement upwards or downwards (e.g. every year there are more HIV infections). When discussing statistics, a trend often describes an apparent difference between results that is not statistically significant. 

mathematical models

A range of complex mathematical techniques which aim to simulate a sequence of likely future events, in order to estimate the impact of a health intervention or the spread of an infection.

representative sample

Studies aim to give information that will be applicable to a large group of people (e.g. adults with diagnosed HIV in the UK). Because it is impractical to conduct a study with such a large group, only a sub-group (a sample) takes part in a study. This isn’t a problem as long as the characteristics of the sample are similar to those of the wider group (e.g. in terms of age, gender, CD4 count and years since diagnosis).

Young women in sub-Saharan Africa have very high rates of HIV infection. In Zimbabwe, incidence of HIV among women aged 15-24 years is twice as high as that observed in men of the same age group.

Modelling studies have shown that sexual relationships with older men – or age-disparate relationships – could be a major reason why young women have such high HIV incidence rates. This is because HIV prevalence is higher among older men than younger men, meaning that young women in relationships with older men are more likely to be exposed to HIV.

However, the precise impact of age-disparate relationships on HIV risk is not fully understood.

An international team of investigators therefore examined data collected between 1998 and 2013 in eastern Zimbabwe to see if having an older male sexual partner was associated with an increased risk of infection with HIV for women aged between 15 and 24 years.

Women included in the study were HIV-negative at baseline and completed at least two follow-up visits. At each visit, social and economic data were collected and if sexually active, participants were asked about the age of their most recent sexual partner.

Relationships were categorised as age-homogenous if there was a 0-4 years difference; intra-generational if the difference was between 5-9 years; and inter-generational if the partner was at least 10 years older.

The investigators performed a set of analyses to see if age and other social, economic and sexual risk factors were associated with an increased risk of HIV incidence.

A total of 3082 women met the inclusion criteria and contributed 3746 study visits. There were 126 new HIV infections during 8777 person-years of follow-up, a rate of 1.43 per 100 person-years.

In all, 2262 study visits involved women who reported sexual activity, and 1734 of these yielded data on the age of the most recent sexual partner.

Just over a fifth of women (21%) had a partner who was ten or more years older and 45% had a partner who was 5-9 years older. These proportions remained steady during the study. HIV prevalence in men declined over time, but remained constantly higher among men aged 30 years and older.

Age difference was associated with higher HIV incidence among younger women. Each one-year difference in age increased the risk by 5% (aHR = 1.05 per year; 95% CI, 1.01-1.09).

Incidence was higher in inter-generational relationships (2.56 per 100 person-years) compared to age-homogeneous relationships (1.49 per 100 person-years).

Similar patterns were found when the investigators took into account factors such as area of residence (urban, peri-urban, rural), household income, wealth and orphanhood.

Women with a secondary education were significantly less likely to report an inter-generational relationship compared to women with lower levels of education (aOR = 0.49; 95% CI, 0.36-0.68).

An inter-generational sexual relationship was associated with the older male having a concurrent sexual partner (aOR = 2.59; 95% CI, 1.81-3.70), and there was a trend for this to be associated with higher HIV incidence among young women (aHR = 1.74; 95% CI, 0.96-3.17).

The association between inter-generational relationships did not change as antiretrovirals were rolled out across Zimbabwe.

“As far as we are aware, this is the first study to demonstrate an association between increasing partner age and HIV incidence in a representative general-population sample covering pre- and post-ART [antiretroviral therapy] periods,” note the authors. “The results from this study provide evidence that age-disparate relationships can be an important contributor to the continued high HIV incidence rates in adolescent girls and young women in sub-Saharan African populations.”


Schaeffer R et al. Age-disparate relationships and HIV incidence in adolescent girls and young women: evidence from a general-population cohort in Zimbabwe. AIDS, online edition, 2017. DOI: 10.1097/QAD.0000000000001506