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Background: The prevalence of hepatitis C in people who use injectable drugs along the USA–Mexico border is very high (>90%). In 2019, the Mexican government committed to providing hepatitis C treatment with priority for people who use injectable drugs, people living with HIV, and people living in prison or jail, yet the immediate plan for prioritized treatment allocation and rollout remained unclear prior to the COVID-19 pandemic. Understanding which prevention and intervention strategies and at what level of scale-up can achieve the WHO goal of 80% incidence reduction by 2030 along the border is needed. Methods: We adapted our previously published dynamic, deterministic model of hepatitis C transmission among people who use injectable drugs to determine the direct-acting antiviral treatment allocations in combination with harm-reduction interventions (opiate agonist therapy and needle and syringe programmes) needed to achieve the WHO elimination goal between 2021 and 2030. Our model is calibrated and parameterised to epidemiological data from Ciudad Juarez (where approximately 10 000 people who use injectable drugs reside and the seroprevalence of hepatitis C among people who use injectable drugs is 92%) and minimal harm reduction. Findings: To reduce hepatitis C incidence by 80% between 2021 and 2030, 910 direct-acting antiviral treatments per 10 000 people who use injectable drugs in Ciudad Juarez per year are needed. Overall, fewer treatments are required if combined with harm reduction. If opiate agonist therapy and needle and syringe programmes are scaled-up to 50%, approximately 30–40% fewer people who use injectable drugs would need to be treated each year (650 direct-acting antiviral treatments per 10 000 people who use injectable drugs per year). Between 2021 and 2030, using direct-acting antivirals alone, an estimated total of 8190 people who use injectable drugs in Ciudad Juarez would need to be treated, compared with 6255 (nearly 25%) fewer people who use injectable drugs overall if treatment is scaled-up alongside 50% of opiate agonist therapy and needle and syringe programmes combination intervention coverage. Interpretation: Hepatitis C treatment with direct-acting antivirals should be prioritised for people who use injectable drugs along the USA–Mexico border and progress towards hepatitis C elimination should be monitored. Regional hepatitis C micro-elimination among people who use injectable drugs could be possible if national treatment allocations are prioritised and distributed to people who use injectable drugs as planned and in the presence of essential harm-reduction programmes. Funding: National Institutes of Health; Fogarty International Center grant D43TW009343; and National Institute of Allergy and Infectious Diseases and National Institute on Drug Abuse grant R01AI147490.

Summary: Background: A considerable proportion of people who inject drugs are unstably housed. Although unstable housing is associated with HIV and HCV infection among people who inject drugs, its contribution to transmission is unknown. We estimated the global and national proportions of incident HIV and HCV infections among people who inject drugs attributed to housing instability from 2020 to 2029. Methods: In this modelling study, we developed country-level models of unstable housing and HIV and HCV transmission among people who inject drugs in 58 countries globally, calibrated to country-specific data on the prevalences of HIV and HCV and unstable housing. Based on a recently published systematic review, unstably housed people who inject drugs were assumed to have a 39% (95% CI 6–84) increased risk of HIV transmission and a 64% (95% CI 43–89%) increased risk of HCV transmission. We used pooled country-level estimates from systematic reviews on HCV and HIV prevalence in people who inject drugs. Our models estimated the transmission population attributable fraction (tPAF) of unstable housing to HIV and HCV transmission among people who inject drugs, defined as the percentage of infections prevented from 2020 to 2029 if the additional risk due to unstable housing was removed. Findings: Our models were produced for 58 countries with sufficient data (accounting for >66% of the global people who inject drugs population). Globally, we project unstable housing contributes 7·9% (95% credibility interval [CrI] 2·3–15·7) of new HIV infections and 11·2% (7·7–15·5) of new HCV infections among people who inject drugs from 2020 to 2029. Country-level tPAFs were strongly associated with the prevalence of unstable housing. tPAFs were greater in high-income countries (HIV 17·2% [95% CrI 5·1–30·0]; HCV 19·4% [95% CrI 13·8–26·0]) than in low-income or middle-income countries (HIV 6·6% [95% CrI 1·8–13·1]; HCV 8·3% [95% CrI 5·5–11·7]). tPAFs for HIV and HCV were highest in Afghanistan, Czech Republic, India, USA, England, and Wales where unstable housing contributed more than 20% of new HIV and HCV infections. Interpretation: Unstable housing is an important modifiable risk factor for HIV and HCV transmission among people who inject drugs in many countries. The study emphasises the importance of implementing initiatives to mitigate these risks and reduce housing instability. Funding: National Institute for Health Research and National Institute of Allergy and Infectious Diseases and National Institute for Drug Abuse.