Table 2.
Model parameters and calibration data common to comparator and intervention
| Parameter | Value | Source |
|---|---|---|
| HCV transmission | ||
| chronic HCV prevalence in 2016 | 41% (36%–46%) | Adjusted from 55% seroprevalence (50%–60%)44 assuming some have spontaneously cleared their infection. |
| risk ratio for HCV transmission and acquisition while on OST | 0.40 (0.22–0.75) | 23 |
| risk ratio for HCV transmission and acquisition while homeless | 2.10 (1.41–3.24) | 24 |
| HCV transmission rate when not homeless or on OST | varied to give HCV prevalence in 2016 | Assume HCV prevalence is stable in the population of PWID. |
| Testing | ||
| percentage of current injectors diagnosed for standard of care | 57% (52%–62%) | 44 |
| factor increase in testing rate in OST compartment compared with not being on OST | 1.22 (1.10–1.35) | Data analysed from needle exchange surveillance initiative in Scotland.25 |
| testing rate in non-OST and ex-injector compartments | varied to give proportion diagnosed shown above (57%) | Assume proportion of injectors diagnosed is stable. |
| Engagement and treatment | ||
| engagement rate from diagnosis for standard of care pathway in all compartments | 0.13 (0.09–0.18) per year | Proportion 0.12 (0.08–0.17) engaged in treatment before intervention (data from HepFriend study initial interviews). Assume this is all in the last year to get yearly rate. |
| treatment rate from engaged per year for standard-of-care pathway in all compartments | 0.34 (0.17–0.59) | Treatment rate among engaged patients from recent UK HepCATT study.18 |
| percentage of treatments that attain SVR | 93% (88%–97%) | SIMPLIFY trial among PWID.26 |
| transition rate from treatment to SVR per year | 1/(12/52) | Treatment assumed to last 12 weeks.26 |
| Demographics | ||
| population size of injectors | 16700 (12525–20875) | Operational Delivery Network Profile Tool developed by PHE.21 |
| cessation rate of injectors | 0.088 (0.053–0.125) per year | Assume mean duration 11 (8–15) years in UK.45 Assumes OST does not affect cessation rate. |
| rate of initiation of injecting | used to fit population size estimates | Assume stable population size. |
| percentage of injectors that are homeless | 33% (26%–40%) | Unlinked anonymous monitoring survey data for London.22 |
| transition rate (not homeless to homeless) | varied to give the sampled proportion of injectors that are homeless | Assume stable proportion of homeless individuals in the population. |
| transition rate (homeless to not homeless) | 1.16 (0.83–1.64) per year | Scottish cohort study found 78/145 homeless PWID were no longer homeless after 8 months.46 Transition probability sampled from β distribution (α=78, β=67) and converted into yearly rate. |
| percentage of PWID on OST | 78% (73%–83%) | Unlinked anonymous monitoring survey data for London.22 |
| transition rate (on OST to not on OST) | 1.48 (1.00–2.98) per year | Duration on OST was 8 months (4–12 months) in a cohort of PWID in UK.47 |
| transition rate (not on OST to on OST) | varied to give proportion of PWID on OST | Assume proportion of PWID on OST is stable. |
| current injector and homeless mortality | same as drug-related death rate | Meta-analysis of mortality in homeless individuals, prisoners and individuals with substance use disorders gave similar mortality ratios for individuals with drug use disorders and individuals who are homeless.48 |
| ex-injector death rate | 0.023 (0017–0.032) per year | 1/(life expectancy–age at initiation of injecting–injecting duration). |
| ex-injector homeless death rate | 0.043 (0.026–0.089) per year | Calculated as for ex-injector death rate with 19 (16–22) years less life expectancy than those who are not currently homeless.38 |