Identifying priority populations for HIV interventions using acquisition and transmission indicators: a combined analysis of 15 mathematical models from ten African countries

Background:
Characterising disparities in HIV infection across populations by gender, age, and HIV risk is key information to guide intervention priorities. We aimed to assess how indicators measuring HIV acquisitions, transmissions, or potential long-term infections influence estimates of the contribution of different populations to new infections, including key populations (KPs, including female sex workers (FSW), their clients, men who have sex with men).

Methods:
Using 9 models representing 15 different settings across Africa, we evaluated four indicators: I1) acquisition indicator measuring the annual fraction of all new infections acquired by a specific population, I2) direct transmission indicator measuring the annual fraction of all new infections directly transmitted by a specific population, I3) 1-year and I4) 10-year transmission population-attributable fractions (tPAFs). tPAFs measure the fraction of new infections averted if transmission involving a specific population was blocked over a specific time period. We compared estimates of the four indicators across 7 populations and 15 settings and assessed if the contribution of specific populations is ranked differently across indicators for 10 settings.

Findings:
Indicators identified distinct priority populations as the largest contributors: The acquisition indicator (I1) identified women aged 25+ years outside KPs as contributing the most to acquired infections in 8/10 settings in 2020, but to direct transmissions (I2) in only two settings. In 6/10 settings, the 10-year tPAFs (I4) identified non-KP men aged 25+ years and clients of FSW as the largest contributors to HIV transmission. Notably, non-KP women aged 15-24 years acquired (I1) more infections in 2020 (median of 1·7-fold across models) than they directly transmitted (I2), while non-KP men aged 25+ years and clients of FSWs transmitted more infections than they acquired in all but one model (median: 1·4 and 1·6-fold, respectively). Estimates of the 10-year tPAFs accounting for transmission in the long term were substantially larger than the direct transmission indicator for all populations, especially for FSW (median: 2·0-fold).

Interpretation:
Indicators that reflect HIV acquisitions and transmissions over the short and long term can be utilised to capture the complexity of HIV epidemics across different populations and timeframes. The added nuance would improve the effectiveness of the HIV prevention response across all populations at risk.