The disparity in disease burden among PWID compared to their population size has been difficult to quantify. Although the Centers for Disease Control and Prevention (CDC) routinely uses population data from the Census Bureau to calculate disease rates by selected demographic categories (e.g., sex, race/ethnicity, and age at diagnosis) [3], [5] no census data are available for the number of PWID in the U.S. and rate calculations require this number for the denominator. Rates allow for comparison among subgroups and over time. Several methods have been used by various countries to measure the size of populations of PWID, including: 1) the capture-recapture method, using data collected from the population at risk; 2) the multiplier method, based on existing data; and 3) the network scale-up method, based on data collected from the general population [6]. For the U.S., multiple data sources have been compiled to estimate the population size of PWID among the nation as a whole and for large metropolitan areas [7]–[10]. While these estimates are informative, they are based on past-year behavior, which is not the most relevant time
