Community coverage and HIV prevention: assessing metrics for estimating HIV incidence through syringe exchange.

BACKGROUND: Evaluations of syringe-exchange programme effectiveness that attempt to measure "coverage" by determining the percentage of the at-risk population reached by a programme are insufficient since programmes must provide syringes on a continual basis. Determining the relationship between the extent of programme coverage and its impact (i.e., reductions in disease risk or incidence) is complicated by the lack of controlled trials with cohorts of drug users and instead has to be estimated by imputation, mathematical modelling, or ecological data analysis. This report offers an approach to determine community-wide impact and discusses the limitations of that approach.
METHODS: Easily created programme tracking data were maintained by exchanges in New Haven, CT, USA and Chicago, IL, USA. Data compiled by month quantified the number of unique participants visiting syringe-exchange programmes and the number of syringes distributed. "Coverage", defined either as the percentage of individuals reached or percentage of community syringe need met, was estimated by incorporating measures of the size of the injector population or injection frequency. These measures of coverage are placed in the context of changing programme operations to estimate the effect of these changes on coverage. Finally, data on AIDS cases from New Haven and Chicago were used to estimate the community-wide impact of syringe exchange.
RESULTS: Two mobile syringe-exchange programmes operated with very different exchange policies. Programme data revealed that coverage of individuals rarely exceeded 10% and was higher in New Haven than in Chicago. On the other hand, coverage measured as the percentage of syringe need met was higher at the Chicago exchange that employed the less restrictive policy. The impact of syringe exchange in the two cities was measured by comparing subsequent AIDS cases. The relative reduction in injection-related AIDS cases as a function of all new AIDS diagnoses was 21.7% in New Haven and 41.4% in Chicago.
CONCLUSIONS: A modest investment in the collection of programme data can yield reliable and interpretable information on the extent of programme reach and retention. Limitations to the approach result from the ecological nature of the data and from the need to use data from outside the programme that may be less reliable. For the cases presented here, coverage rates will vary as a function of the programme policies; however, even modest coverage rates - well below those recommended by UNAIDS - can have significant impacts on HIV epidemics. Restrictive policies appeared to increase the coverage if measured only by the proportion of monthly participants and not by the proportion of syringe need met by a programme. More generally, programmes can collect programmatic data and some rapid assessment data (estimates of IDU population and injection frequency) to estimate of the impact of their programmes.