The population proportions of PWID in the age group 18–24 years were applied to the population aged 13–24 years and the population proportions of PWID in the age group 50–64 years were applied to the population aged 50 years or older. Because smaller percentages of persons in both age groups (13–17 and 65 years or older) are less likely to have ever injected drugs than those aged 18–24 years and 50–64 years, respectively [20], this may result in an over-estimate of the number of PWID and thus an under-estimate of the rates of HIV infection.
