| Scenario | Brief description of scenario | Details of how to derive model inputs from data ranges in table 2 | |||||
| Proportion in risk group | Number IDUs share with per month | Number of syringe sharing incidents per IDU per month | |||||
| Low | High | Low | High | Low | High | ||
| Scenario 1 | For each model input, the original distributions for each survey parameter in table 2 were sampled and each parameter was estimated as in adjacent columns. Scenario directly used data estimates for the proportion of IDUs reporting syringe sharing and average number of people an IDU shares with per week stratified by where they obtained their needles/syringes (see table 2), and averaged the number of needle/syringe sharing incidents between them (product of injecting frequency and % of injections shared). | A0*C0 | A1*C1 | 4*E0 | 4*E1 | B0*D0/(4*E0) | B1*D1/(4*E1) |
| Scenario 2 | Sampling and proportion in each group as scenario 1. Scenario split needle/syringe sharing partners by whether they were “strangers” (an IDU they report not having shared with before) or not, and assumed the same “non-strangers” were shared with each week but that “strangers” were different. Only one syringe needle/sharing incident assumed to occur with each “stranger” and remaining sharing incidents occurred with “non-strangers”. | A0*C0 | A1*C1 | 4*E0*F0 and E0*(1−F0) | 4*E1* F1 and E1*(1−F1) | 1 and B0*D0−1 | 1 and B1*D1−1 |
| Scenario 3 | Sampling and proportion in each group as scenario 1. Scenario assumes only “stranger” needle/syringe sharing incidents are important for HCV/HIV transmission in Rawalpindi, with other sharing incidents being ignored. Number of “strangers” per month calculated as scenario 2, and number of sharing incidents with each assumed to be 1. | A0*C0 | A1*C1 | 4*E0*F0 | 4*E1* F1 | 1 | 1 |
| Scenario 4 | Sampling and proportion in each group as scenario 1. Scenario same as scenario 3, but assumes the risk of becoming infected by a “stranger” is greater than suggested by data (assumed 0–100% greater). This accounts for the likely under-reporting of needle/syringe sharing with “strangers” and that an infected IDU is likely to infect their regular needle/syringe sharing partner. Number of strangers an IDU shares needle/syringes with per month calculated as in scenario 2 but multiplied by 0–100% (defined as J0 and J1). | A0*C0 | A1*C1 | 4*E0*F0*J0 | 4*E1* F1*J1 | 1 | 1 |
| Scenario 5 | Sampling as scenario 1. Scenario used to explore whether HIV epidemic could be concentrated among high-risk IDUs, as suggested by 100% HCV prevalence among HIV-infected individuals. Scenario splits IDUs by whether do not share, report needle/syringe sharing with “strangers” in the last week (high-risk), or do not (low-risk). Range for proportion ever sharing (G) or sharing with “strangers” in last week (H) extended to 35–95% and 6%–20%, respectively, to account for accidental syringe sharing, reporting bias and equipment sharing. Large ranges used to make scenario more exploratory. Number of “strangers” shared with per month for high-risk IDUs was survey range, whereas a wide range was used for low-risk IDUs but always <1 per week. Syringe sharing with “non-strangers” assumed to be low risk and ignored. | G−H | H | Assumed range | I | 1 | 1 |
HCV, hepatitis C virus; IDU, intravenous drug user.
A to I are as defined in table 2.