This chapter shows how successive yellow fever vaccines, conceived as complex sociotechnical constructs, have been involved in the construction of the Brazilian nation state. Three distinct periods in the country’s political history are distinguished: the patriarchal oligarchic state (1822-1930), the national developmentalist state (1930-80), and the state which has since then oscillated between liberal dependency and national interventionism. The successful campaigns against yellow fever run by Oswaldo Cruz formed the backbone for the founding myth of scientific public health and medicine in Brazil. The trajectory of the yellow fever vaccine manufactured at the Oswaldo Cruz Institute, which eventually became the biggest producer worldwide, coincides with economic, welfare, and labour policies that principally benefited urban groups. Rural populations would be the main recipients of the yellow fever vaccine, and it became an important component when national agencies tackled endemic diseases in the interior. Immunisation programmes have helped strengthen the country’s health system, disseminating a culture of prevention. The social mobilisation achieved by the yellow fever and other vaccination campaigns led to new relationships between communities and health services.