# Evaluating the biomedical and behavioral drivers of HIV incidence decline in adolescent girls and young women in Uganda: A mathematical modeling study

## TL;DR

This study finds that both biomedical interventions and delayed sexual debut contributed to a significant drop in HIV cases among young women in Uganda.

## Contribution

The study quantifies the combined and individual impacts of biomedical and behavioral factors on HIV incidence decline in adolescent girls and young women.

## Key findings

- HIV incidence among 15–24-year-old women in Uganda declined by 71% from 2000 to 2019.
- Delays in age at first sex had the largest impact on reducing infections in adolescent girls.
- ART scale-up and VMMC also significantly contributed to incidence declines, with additive effects when combined with behavioral changes.

## Abstract

HIV incidence among adolescent girls and young women (AGYW) in eastern and southern Africa has declined substantially over the past two decades. These declines are often attributed to biomedical HIV prevention strategies, though concurrent changes in sexual behavior may also contribute. We evaluated the contributions of biomedical and behavioral drivers to historical incidence decline in AGYW and projected their impact on incidence trajectories over the next 30 years.

We conducted a mathematical modeling study using data from the Rakai Community Cohort Study (RCCS), an open, population-based cohort of adults aged 15–49 years in 30 communities in Rakai, Uganda. We used an agent-based HIV-1 transmission model calibrated to cohort data to estimate HIV incidence trends among AGYW, aged 15–24, and to quantify the independent and combined effects of antiretroviral therapy (ART), voluntary medical male circumcision (VMMC), and changes in age at first sex (AFS).

HIV incidence among women aged 15–24 declined by 71% between 2000 and 2019, from 1.57 to 0.45 per 100 person-years, representing the largest decline across female age groups in the cohort. Increasing AFS over the study period (by approximately 3 years in women and 2 years in men) was the largest contributor to incidence declines among adolescent women aged 15–19, averting 17% of cumulative infections between 2000 and 2020 and 37% between 2000 and 2050. Among women aged 20–24, ART scale-up had the greatest impact, averting 13% of infections by 2020 and 43% by 2050. VMMC contributed modestly to historical declines but had larger projected effects over longer time horizons. ART, VMMC, and delays in AFS acted additively to reduce HIV incidence among AGYW. Study limitations include reliance on self-reported sexual behavior and the use of a mathematical model that cannot capture all real-world sexual network dynamics.

Both biomedical HIV interventions and broader behavioral changes contributed to declines in HIV incidence among AGYW. Sustaining continued incidence declines in young women will require maintaining both the protective changes in sexual behaviors and effective biomedical interventions.

Adolescent girls and young women in eastern and southern Africa have historically faced the highest risk of acquiring HIV.

Over the past two decades, HIV infections among young women in Uganda have declined sharply, but the reasons for this decline are not fully understood.

Both biomedical interventions (such as HIV treatment and male circumcision) and changes in sexual behavior (such as later age at first sex) occurred during this period, making it important to understand their individual and combined effects.

We used a mathematical model calibrated to long-term population data from southern Uganda to examine changes in HIV incidence between 2000 and 2019.

HIV incidence among young women aged 15–24 declined by more than 70%, with the largest reductions occurring among adolescents aged 15–19.

Delays in the age at which young people first had sex, along with expanded access to HIV treatment and male circumcision, each contributed to these declines, and their effects were largely additive.

Our results show that both biomedical HIV interventions and broader social and behavioral changes have played important roles in reducing HIV infections among young women.

Sustaining progress toward HIV epidemic control will likely require continued investment in HIV treatment and prevention alongside efforts that support education and social conditions associated with delayed sexual debut.

This study relied on self-reported sexual behavior and mathematical modeling, which may not capture all real-world complexities, and future changes in HIV programs or social conditions could alter long-term projections.

Using mathematical modeling, Adam Akullian and colleagues estimate that increasing age at first sex is a key driver of the decline in HIV incidence among young women in the Rakai Community Cohort Study in Uganda.