Rare disease definitions across regulatory and reimbursement systems: policy objectives and implications for patient access.

Rare disease definitions are used for different purposes across epidemiological, regulatory, and reimbursement settings, yet no globally accepted standard exists. Regulatory authorities commonly adopt prevalence-based definitions to facilitate market entry and incentivise innovation, whereas health technology assessment agencies prioritise value for money and health system sustainability, often introducing narrower categories such as "ultra-rare" diseases to guide reimbursement decisions. This policy comment examines how definitions used across regulatory and reimbursement contexts reflect different institutional mandates and explores the implications of variation between them. Differences in definitions can influence eligibility for regulatory incentives, reimbursement pathways, and ultimately patient access to treatment. At the same time, prevalence-based frameworks may create unintended incentives to classify conditions as rare and raise questions about how emerging challenges, such as precision medicine and indication expansion should be accommodated. We argue that policy discussions should move beyond identifying a prevalence threshold and instead focus on the purpose the eligibility criteria are intended to serve.