Should rare and ultrarare diseases be treated differently in health coverage decisions?

BACKGROUND: Health technology assessment (HTA) is widely used to inform coverage and reimbursement decisions in health systems by integrating evidence on clinical effectiveness, safety, cost-effectiveness, and broader social considerations. However, the application of conventional HTA frameworks to rare diseases remains challenging due to limited clinical evidence, substantial uncertainty, and ongoing debates regarding the appropriateness of standard evaluation criteria. In this context, this study aimed to identify and synthesize additional criteria and their underlying justifications proposed and/or applied in HTA-based decision-making for rare diseases, and to examine the extent to which these criteria have been adopted by HTA agencies across countries. METHODS: The protocol was registered in PROSPERO (CRD420251115351). We conducted an overview of reviews in accordance with the Cochrane Handbook and the PRIOR statement, complemented by a structured grey literature search of health technology assessment agencies and Ministries of Health affiliated with INAHTA and RedETSA. Systematic and scoping reviews, as well as institutional documents addressing criteria used in coverage decisions for rare diseases, were included. Findings were synthesized narratively, with criteria grouped into adapted standard and additional categories, and implementation patterns examined across different health system types. RESULTS: Seventeen reviews and multiple institutional documents from eleven countries were included. Seven standard health technology assessment criteria and ten additional criteria were identified. Most jurisdictions reported adaptations of conventional health technology assessment approaches to address uncertainty and ethical concerns associated with rare diseases; however, only a limited number had operationalized a comprehensive set of additional criteria. Disease severity and therapeutic advantage were the most frequently applied additional criteria across countries, whereas treatment characteristics and risk-sharing mechanisms were inconsistently reported. Evidence specific to ultrarare diseases was scarcer, suggesting that decision-making for ultrarare conditions likely relies on criteria applied to rare diseases. CONCLUSIONS: Health technology assessment for rare and ultrarare diseases is characterized by selective and pragmatic adaptations rather than the use of unified alternative frameworks. REGISTRATION: PROSPERO CRD420251115351.