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Research problem and specific questions: This project addresses the public health challenge of understanding how migration status, social stratification, and gender influence health outcomes among people who inject drugs (PWID) in Sweden. Despite Sweden’s commitment to harm reduction, disparities persist, particularly among migrants and socioeconomically disadvantaged individuals. Key research questions: 1. How do injection risk behaviors differ by migration status, gender, and social stratification at enrollment and over time? 2. How do migration status, gender, and social stratification influence access to needle and syringe program (NSP) services and coverage across Sweden? Data and method: For quantitative analysis, the project will use data from the Swedish NSP quality register, InfC are NSP, covering 2013-2025. It is used by 20 of 21 Swedish regions, capturing 98% of all PWID in Swedish NSP (N=13,000). Methods such as propensity score matching (PSM), marginal structural models (MSM), and difference-in-differences (DiD) will be used to strengthen findings and inform policy recommendations. Semistructured interviews will be used for qualitative analysis. Societal relevance and utilisation: This research includes groups often treated as low priority, ignored, and carrying multiple stigmas, both as immigrants and injecting drug users. Women in these categories are extremely vulnerable, facing compounded challenges due to hepatitis, HIV, mental illness, and social exclusion. They are difficult to reach with harm reduction interventions. The results are relevant for actors such as the Public Health Agency, decision-makers, healthcare, and service providers, who need to implement evidencebased guidelines to reach this vulnerable and often care-demanding group. By focusing on migration, social stratification, and gender, the project aims to reduce disparities in access to healthcare and promote the inclusion of marginalized groups. Plan for project realisation: The project will be conducted over three years (2026–2028). Data collection, analysis, and manuscript preparation will be carried out according to the work plan. Findings will be disseminated through published manuscripts, scientific conferences, and meetings with stakeholders such as the Public Health Agency and regional NSP coordinators. Principal Investigator with expertise in harm reduction and advanced quantitative methods, will collaborate with different experts to ensure the project’s success.

Estimating the size and coverage of hard-to reach and stigmatized (“hidden”) populations such as people who inject drugs (PWID) remains a major challenge for public health e.g. prevention, policy and programming. Today, the estimated number of PWID in Sweden varies between 8,000-10,000 that was calculated using multiplier methods, which combine data from needle exchange programs, treatment entries, and survey data to estimate the hidden population of PWID. This pilot study aims to integrate brief, standardized questions in routine clinical data collection at needle and syringe program (NSP) sites (harm reduction services) in Sweden to help estimate network size, thereby providing necessary data for more precise service demand and program coverage estimations. The goal of this project is to assess the feasibility and utility of collecting such data for future use in population size estimation and service coverage analysis that could be applied also to other hard-to reach populations. The project will provide proof-of-concept for a scalable, low-burden data collection method that could be expanded nationally. The results will support improved epidemiological surveillance and more targeted harm reduction planning. Specific Aims: 1) To develop and pilot test a short module of network size questions tailored for PWID in needle and syringe exchange harm reduction programs (NSP)

2) To assess the feasibility and acceptability of integrating network questions into NSP service routines. 3) To evaluate the potential of collected network data for estimating PWID population size and NSP coverage (using the network scale-up method or other methods).