Brief report: Aging of the HIV epidemic in a longitudinal population cohort and its implications for the understanding of long-term HIV prevalence trends

BACKGROUND: In settings with widespread antiretroviral therapy (ART), improved survival and changing HIV incidence patterns can shift the age distribution of people living with HIV. Such demographic changes may complicate interpretation of crude prevalence trends, which can reflect population aging rather than true epidemiological shifts. SETTING: Population-based HIV surveillance by the Africa Health Research Institute in rural KwaZulu-Natal, South Africa, among adults aged 15-54 years (2007-2024). METHODS: We assessed changes in age-specific HIV prevalence and population age structure over time. Crude prevalence trends were compared with directly age-standardized estimates using four reference populations (2007, 2017, 2024, and the overall mean age distribution). Temporal trends were analyzed using generalized estimating equation logistic models adjusted for age group and sex to account for repeated observations. RESULTS: HIV prevalence increasingly shifted toward older adults, alongside gradual aging of the surveillance population. Crude prevalence rose from 22.3% (95% CI 21.4-23.2) in 2007 to 35.0% (34.1-36.0) in 2016, remaining broadly stable through 2024 (33.3%; 32.3-34.2). After age standardization, prevalence declined by approximately 4-6 percentage points after 2016 across all reference populations. Adjusted models showed similar patterns: odds of HIV infection peaked in 2016 (OR 1.55; 1.48-1.62 vs 2007) and decreased to 1.35 (1.28-1.42) by 2024. CONCLUSIONS: HIV prevalence is increasingly concentrated at older ages in this cohort, reflecting demographic aging in the ART era. Age standardization clarifies longitudinal prevalence trends by distinguishing demographic shifts from underlying epidemiological change.