Chapter six argues that by the end of 1936, the Irish Free State had come close to incepting an operational national anti-diphtheria immunization scheme. This is a noteworthy achievement, as state-backed anti-diphtheria schemes were not introduced as an intervention against this pressing public health issue in the rest of Europe until 1938 and were only pursued with any vigour when wartime conditions exacerbated the problem from 1940 onwards. If it had progressed unimpeded, the Free State intervention seemed destined to eliminate diphtheria, and to become the first established national childhood immunization programme in Europe. However, the death of Siobhán O’Cionnfaola in April 1937, and the subsequent controversy surrounding the Ring incident, asked serious questions of active immunization and ultimately undermined vaccine confidence among parents, practitioners, and politicians. This chapter will evaluate the impact of the Ring controversy and the social, political and medical implications left in the wake of the incident.