Utilising integrated bio-behavioural surveillance (IBBS) to investigate declining hepatitis C antibody prevalence among people who inject drugs in the Australian Needle and Syringe Program Survey.

Background: Prevalence of hepatitis C virus (HCV) antibody (Ab) on dried blood spot (DBS) samples in the Australian Needle and Syringe Program Survey (ANSPS) decreased nationally from 57 % in 2015 to 32 % in 2022. We aimed to investigate potential explanations for this decline.

Methods: Changes in DBS HCV Ab prevalence were investigated by redefining positive cases as those with those with either a positive HCV Ab test result or a self-reported history of ever having HCV treatment (modified prevalence), examining HCV Ab prevalence by birth and age cohorts, and assessing trends in key risk behaviours.

Results: Overall prevalence of DBS HCV Ab declined rapidly and significantly from 57 % in 2015 to 32 % in 2022 (p < 0.001) however modified HCV Ab prevalence remained stable over time (85 % and 88 % in 2015 and 2022, respectively, p = 0.357). The proportion of participants with negative HCV Ab and self-reported HCV infection increased from 20 % in 1995 to 40 % in 2022 (p < 0.001) and the proportion with negative HCV Ab and lifetime HCV treatment increased from 3 % in 1999 to 67 % in 2022 (p < 0.001). We also observed a decreasing trend in DBS HCV Ab prevalence in all birth and age cohorts with a noticeable acceleration in the decline commensurate with the advent of HCV DAA treatment. A long-term decreasing trend was also observed for key risk behaviours (p < 0.001) however the short-term trend was not significant for recent receptive syringe sharing.

Conclusion: The temporal decline in HCV Ab prevalence appears related to reduced sensitivity of DBS HCV Ab detection with viral clearance following treatment. Since 2016, HCV treatment uptake has increased markedly including among people who inject drugs. In this context, continuing to monitor HCV Ab prevalence by DBS testing is problematic, with a shift to surveillance of active infection the most relevant to guide policy and practice in this setting.