Estimated Number of People Who Inject Drugs (PWID) in the US

"We estimated nearly 3.7 million people, or 1.5% of the US adult population, injected drugs in 2018. This estimate is more than 5 times the most recent US estimate of ∼774,000 from 2011 [25]. Much of this increase is likely attributable to increases in IDU, but it is important to consider methodological differences in the creation of this 2018 estimate vs the 2011 estimate. The 2011 estimate was based on self-reported IDU among respondents to household surveys [26], but the present estimate combines available data on substance-specific overdose deaths and treatment admissions with cohort and cross-sectional data collected from known PWID. Applying the same data sources and analytic methods used for the 2018 estimate to 2011 yields an estimated 1.3 million PWID in 2011, which suggest the 2018 estimate is closer to 3 times higher than in 2011. By any measure, these estimates suggest the number of PWID has increased substantially in the U.S. during the past decade.

"One of the primary contributions of this estimate is the transparent, replicable nature of the methods described. Overdose data specifically among PWID in the United States continue to be relatively sparse, both in research and surveillance data. We used the best data currently available for each input, which are subject to limitations in some cases given data sparsity. For example, we used the meta-analyzed ratio of fatal to nonfatal overdose among PWID in OECD countries rather than a ratio specific to the United States, which was unattainable given currently available data. The uncertainty associated with this meta-analyzed ratio is reflected in confidence intervals around estimates presented here. Our intention is that, as surveillance systems implemented in the United States in recent years mature [39], resulting data can be used to refine and update this PWID population size estimate.

"Notwithstanding data input limitations, this updated estimate provides a data point for monitoring the US PWID population size over time and can inform strategies to reduce transmission of infectious diseases. In recent years, political will has been building to eliminate HCV and HIV infections in the United States [27, 28]. Both bloodborne infections disproportionately affect PWID but are highly preventable using evidence-based interventions, such as provision of sterile syringes through syringe services programs and substance use treatment [40–43], as well as treatment of prevalent infections with antiretroviral therapy [44] and direct-acting antivirals [45]. Increases in IDU prevalence will threaten the success of elimination strategies for HCV and HIV infections in the absence of concomitant increases in availability of harm reduction services and treatment for both infectious diseases and substance use. These services will need to be substantially scaled up nationally to meet the needs of nearly 4 million people [46].

"In addition to the high burden of infectious diseases, PWID experience preventable mortality and morbidity due to drug overdose. Overall, the rate of overdose deaths increased from approximately 6 per 100,000 persons to 22 per 100,000 persons during 1999–2019 [21], and provisional data indicate the number of overdose deaths increased by another 31% during just 1 year of the pandemic era from March 2020 to March 2021 [24]. During the pandemic era in particular, many questions remain about the extent to which increased overdose mortality rates are attributable to injection initiation vs changes in injection behaviors or the drug supply as well as to disruptions in access to treatment and recovery support services and harm reduction services. These estimates provide a prepandemic baseline and can improve our understanding of potential increases vs changes in pandemic-era injection behavior."