This paper takes a constructivist approach to examining one of the new norms embodied in the recently revised International Health Regulations (IHR). The paper focuses on the provisions that seek to restrain states from applying disproportionate international travel and trade restrictions in response to a disease outbreak occurring in another country. This new norm, which aims to limit unjustified 'additional health measures', has significant implications for state sovereignty. Using the example of the 2009 H1N1 'swine flu' pandemic, the paper examines whether state behaviour and the discourse surrounding that outbreak supports a constructivist contention that a new norm has been created and that most states can be expected to comply with that norm most of the time. We conclude by discussing what the discourse over H1N1 suggests about the extent to which the new norm concerning additional health measures has been internalized by states. © 2012 Taylor & Francis.