Chasing 'vulnerability' across six decades of the Declaration of Helsinki.

The year 2024 marked the 60th anniversary of the World Medical Association's Declaration of Helsinki (DoH). Coincidentally, the WMA published the 8th revision of this landmark document guiding medical research involving human subjects. One of the key changes in this latest revision concerns the notion of vulnerability, which has always been central to the DoH's ethos. The term 'vulnerability' was explicitly introduced in the 5th revision, published in 2000, which lists five vulnerable groups. Subsequent revisions have significantly altered how vulnerability is portrayed and understood within the document. This article traces the conceptualisation of vulnerability across the various versions of the DoH, culminating in its recently published 8th revision. We explore the underlying principles of each revision and examine how these principles have both influenced and been influenced by broader ethical discourses. Lastly, we address some of the challenges that future revisions must meet to ensure that the document remains internally coherent and practically applicable for researchers and research ethics committees alike.