BACKGROUND: The coronavirus disease 2019 (COVID-19) pandemic may have disproportionally impacted vulnerable groups such as people who inject drugs (PWID) through reduced health care services as well as social changes from pandemic mitigation measures. Understanding how the COVID-19 pandemic and associated mitigation strategies subsequently changed the trajectory of hepatitis C virus (HCV) and human immunodeficiency virus (HIV) transmission is critical to estimating disease burdens, identifying outbreak risk, and developing informed intervention strategies. METHODS: Using behavioral data from the AIDS Linked to the IntraVenous Experience (ALIVE) study, an ongoing community-based cohort of PWID in Baltimore, United States, and an individual-based network model, we explored the impacts of service disruptions combined with changes in social networks and injecting behaviors of PWID on HCV and HIV transmission. RESULTS: Analyses of ALIVE data showed that during the pandemic, there was an acceleration in injection cessation trajectories overall, but those who continued injecting increased the frequency of injection
at the same time, individual drug-use networks became smaller and the probability of injecting with others decreased. Simulation results demonstrated that HCV and HIV prevalence increased from service disruptions alone, but these effects were mitigated when including observed behavior changes in addition. CONCLUSIONS: Model results combined with rich individual behavioral data indicated that pandemic-induced behavioral changes of PWID that lasted longer than service disruptions could have offset the increasing disease burden caused by disrupted service access during the pandemic.