Trends in the prevalence of HIV, HCV, and co-infection and associated factors among young people who inject drugs (PWID) in Catalonia (2008-2019)

People who inject drugs (PWID) face a significantly increased risk of blood-borne virus (BBV) infections, including HIV and hepatitis C virus (HCV), as well as co-infections. To better understand this issue, bio-behavioral surveillance studies were conducted among PWID attending harm reduction facilities in Catalonia, forming part of the comprehensive Integrated HIV/STI Surveillance System since 2008. The goal of this study was to describe the prevalence of HIV, HCV, and HIV-HCV coinfection and associated socio-demographic and behavioural factors among young PWID in Catalonia by gender and to examine trends over time, from 2008 to 2019. We first estimated the prevalence of HIV, HCV, and their co-infection and sample characteristics, and then, we calculated prevalence ratios (PR) and adjusted prevalence ratios (aPR) to identify the variables associated with HIV infection, HCV infection, and HIV-HCV co-infection. We found that the overall prevalence of HIV, HCV and HIV-HCV co-infection in 2019 was 20.1%, 66.2%, and 16.8%, respectively. In multivariable analysis, we found that being female (aPR: 1.55, 95% CI: 1.05-2.30), age (aPR: 1.09, 95% CI: 1.04-1.14), having born outside of Spain (aPR: 1.67, 95% CI: 1.19-2.34), homelessness (aPR: 1.90, 95% CI: 1.34-2.68), daily injection (aPR: 1.46, 95% CI: 1.04-2.04), sharing needles (aPR: 1.93, 95% CI: 1.32-2.82), involvement in sex work (aPR: 1.82, 95% CI: 1.09-3.03), and imprisonment (aPR: 2.54, 95% CI: 1.75-3.67) was associated with increased prevalence of HIV and HCV co-infection, while being a new injector (aPR: 0.64, 95% CI: 0.43-0.93) was associated with decreased prevalence. Overall, this research provides valuable insights into the risks and determinants of HIV and HCV co-infection among young PWID. The study's findings contribute to the existing body of knowledge on blood-borne virus transmission in this population, informing evidence-based strategies for prevention and harm reduction.