[The analysis of complex interventions in public health: the case of the prevention of sexually transmitted infections and blood-borne infections in Montreal].

Objectives: Based on a theory of intervention as a complex action system, analyze collaboration among partners in Montréal's sexually transmitted and blood-borne infections (STBBI) prevention program to identify main operations problems and possible scenarios for change to achieve better outcomes.

Methods: A descriptive study was conducted using three data sources - public policies and programs, system management documents, and interviews with three types of partners. The results were validated with stakeholders.

Results: Five main operations problems affecting the capacity of the system to provide expected services were identified, as well as strategies the partners use to address these.

Conclusion: Two scenarios for system change to increase its effectiveness in achieving program goals are discussed.