Letters and lymph: Jean de Carro and the transnational circulation of Jennerian vaccination, 1799-1810.

This article examines the early continental diffusion of Jennerian vaccination through the activities of Jean de Carro, a Genevan physician established in Vienna. Drawing on published correspondence, including exchanges with Edward Jenner and Alexandre Marcet, as well as contemporary medical writings, it reconstructs the circulation of both information and vaccine lymph across Europe in the first decade following Jenner's discovery. It argues that the spread of vaccination cannot be explained solely by institutional initiatives or state-sponsored campaigns but depended on transnational networks of physicians who exchanged letters, translated texts, and transmitted biological material. Within these networks, De Carro acted as one of the key intermediaries-or scientific broker-facilitating both the circulation of knowledge and the transfer of vaccine lymph. Contemporary evidence indicates that he received vaccine material from Britain and redistributed it through a wider European network, including Geneva, where it was experimentally tested by physicians such as Louis Odier. By situating De Carro within a wider network of cosmopolitan physicians linking London, Vienna, and Geneva, this article highlights the combined importance of epistolary exchange and material circulation in the rapid dissemination of vaccination. It suggests that such networks help explain the remarkable speed with which vaccination spread from Britain to Central Europe and, subsequently, towards the Ottoman Empire and India between 1799 and 1810.