Coverage of HIV prevention programmes for injection drug users: confusions, aspirations, definitions and ways forward.

The concept of coverage first emerged in the 1960s as a key indicator for measuring the proportion of populations that were covered by health care. In the area of HIV interventions among injecting drug users, the term "coverage" has been used for widely different aspects of reach and effectiveness of programmes. This paper reviews an array of ways of thinking about coverage from the scientific and grey literature and discusses methods of estimating coverage, finding that some measures of coverage refer to individuals, others to populations and others to populations covered by services. Nomenclature for these various types of "coverage" are discussed and recommendations are provided for future attempts to measure coverage of harm reduction interventions.