How do treaties change over time? This Article joins a growing body of scholarship focusing not on formal change mechanisms but instead on informal change arising from a treaty’s implementation in practice. Informal implementation is often murky, poorly documented, and may be indistinguishable from noncompliance. Yet it is significant both doctrinally under the Vienna Convention on the Law of Treaties—a set of rules for the formation and operation of treaties—and in its own right, when it does not meet the requirements to be doctrinally relevant. Based on a deep dive into the history of one of the oldest areas of continuous international regulation, infectious disease control, and drawing on insights from scholarship on how domestic contracts, statutes, and institutions change informally over time, I argue that (1) change in informal implementation is often an alternative to formal change pursued by those unable to achieve the latter; (2) the process of informal implementation is akin to a strategic game in which a host of actors struggle to move the practice of a treaty toward their own preferences; and (3) informal implementation-level change has the potential to be vast in scope and can precipitate legislative updates later on.

Understanding that transformative change can originate from the complex, decentralized, and oftentimes opaque world of informal treaty implementation raises new inquiries and impacts long-standing issues in international law. It asks, at the most fundamental level, what exactly written international law is—a blueprint awaiting faithful execution or a departure point for further bargaining? It calls for a more nuanced understanding of compliance—currently, a central preoccupation in the field—since noncompliance can in fact be implementation leaping ahead of the treaty as written. And it invites a normative exploration of whether informal implementation is a cause for concern because it moves treaty making away from highly visible formal processes, or a mechanism to be channeled because it could expand the voices and influence of the disempowered.

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