The 2025 European health data space regulation: implementation challenges in harmonising opt-out rules for reusing health data.

BACKGROUND: In 2025, the European Health Data Space (EHDS) entered into force. A key objective of this landmark European Union (EU) health policy is to facilitate international biomedical research by harmonising privacy rules on data reuse. REFORM CONTENT: A central political disagreement in the preparation of the EHDS concerned whether there should be an opt-in rule for health data- whereby citizens have to explicitly consent for their data to be reused within the EHDS - or if an opt-out rule would suffice. EXPECTED RESULTS: It was finally agreed that the EHDS regulation would include an opt-out rule - meaning that reuse of data for research is possible, unless individuals actively opt-out. This policy decision was accompanied by the promise that European harmonisation on privacy rules for health data reuse was achieved. However, true harmonisation can be reached only if also the implementation of legal rules is coordinated. By providing an overview of how divergencies arose in the operationalisation of similar opt-out rules in previous health data policies from selected European countries, we show that the road to European harmonisation is still long. CONCLUSIONS: We conclude by arguing that true harmonisation of privacy rules for health data reuse requires to pay special attention to the implementation phase of the opt-out rule of the EHDS policy.