The sharing of vaccine was not only a question of collegiality and medical ethics, but a basic requirement for sustaining vaccinations. By transmitting the vaccine to other, nearby regions, one could create reservoirs for retransmission to draw on in case a chain of transmission was interrupted. Kasahara Hakuō 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 that his goal was to bring this “incomparable great method to the entire country, without any 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 this aspiration, one might expect Hakuō to have liberally shared the vaccine, but this was not actually the case. The town doctor tried to tightly control sharing and carefully documented the path of the vaccine. This pathway follows a number of early instances in which Hakuō shared the vaccine with physicians outside his domain territory.