Resumen

BACKGROUND: Tijuana has one of the largest populations of people who inject drugs (PWID) in Mexico, and sex-work is quasi-legal for female sex workers (FSW). Tijuana also has a large population of men who have sex with men (MSM), meaning there are overlapping key populations contributing to the HIV epidemic. We model the extent to which HIV transmission is driven by specific key populations and risk behaviours. METHODS: We developed and calibrated an overlapping dynamic HIV transmission model among PWID, FSW, clients of FSW and MSM using data from several completed and on-going studies. Baseline HIV prevalence was 3.5% (2.3-5.1%) in 2012, 2.7% (1.2-5.2%) in 2013 and 17.3% (12.2-23.4%) in 2012 among PWID, FSW and MSM, respectively. We projected the number of HIV infections over 10 years (2019-2029), and the proportion of these attributable to different key populations and sexual or injecting behaviours (defined as the percentage of new infections prevented when these modes of transmission are removed). RESULTS: The model projects overall HIV incidence of 0.4 per 100person-years (95% Credible Interval (95%CrI:0.3-0.6/100pyrs) in 2019 across all key populations modelled. Unprotected sex between men accounted for 56.6% (95%CrI:13.8'82.1%) of all new HIV infections among key populations over 2019-2029, while injecting drug use contributed 31.9% (95%CrI:3.5'82.0%) and unprotected commercial heterosexual sex contributed 15.3% (95%CrI:6.1'22.6%). Non-commercial heterosexual sex contributed <10%. By key population, making all behaviours safe among PWID, FSW, MSM and clients would prevent 52.3% (95%CrI:30.9'87.9%), 17.7% (95%CrI:9.8'24.2%), 57.8% (95%CrI:14.3'82.8%) or 17.7% (95%CrI:8.9'24.0%) of all infections among key populations over 2019-2029, respectively. Among PWID, HIV transmission is due to multiple risk behaviours: 69.9% (15.6'95.2%), 16.6% (2.3'46.7%) and 12.5% (4.7'23.5%) of new infections among PWID over 2019-2029 are due to unsafe injecting, unprotected sex between men and heterosexual transmission between PWID, respectively. CONCLUSIONS: Most HIV transmission among key populations in Tijuana is due to sexual transmission, although injecting drug use still plays a large role. Removing transmission risks due to unprotected sex between men provides the biggest reduction in new infections, indicating this is a key behaviour to prioritize interventions towards.