The sharing of vaccines was not only a question of collegiality and medical ethics, but a basic requirement for sustaining vaccinations. By transmitting the vaccine to nearby regions, one could create reservoirs for retransmission in case the process was interrupted and the vaccine went extinct. Kasahara Ryōsaku was aware of the practical benefits of sharing and also hoped to save as many children as possible by achieving maximum coverage. In early 1850, he wrote in a letter about his goal of bringing this “incomparable great method to the entire country, without omission.” [Hakushinki, 1850, 1/17, kakitori, attached to previous document, Copy of the kakitori to be sent together with the letter to physicians of nearby domains, p. 55-56]
Given these aspirations, one might expect Kasahara to have liberally spread the vaccine to as many locations as possible, but this was not actually the case. The town doctor carefully documented the path of the vaccine and was very selective about the physicians he shared with and allowed to handle the substance. For different reasons, Fukui domain and the vaccinators were interested in maintaining some control over the flow of the lymph from body to body. Because Kasahara hoped to pass on the vaccine to physicians of other domains, he constructed a trans-territorial network of vaccinators, which used control over the flow of medical knowledge to control transmission of the vaccine. This pathway explores the process and conditions of sharing by following a number of early instances in which Kasahara shared the vaccine with physicians outside his domain territory.