Immunization now prevents 2-3 million diphtheria, tetanus, pertussis and measles deaths every year. In this regard, it is considered one of the most successful and cost effective public health interventions. Immunization has many benefits. Moreover, these benefits are reflected both in the immunized individual and in other segments of society. One of these benefits is the externalities that immunity creates. Externality is an important concept in the health economy as it is in the economy. As one of the preventive health interventions, the contribution of immunization, that is, the externalities, is inevitably greater than the contribution to public health and the health economy. The aim of this study is to provide a perspective on the externality of immunization in terms of health economics. In this context, the concept of immunization in the study is explained and the immunization related data are given worldwide. In addition to the positive externality of immunization, it was emphasized that there are negative externalities, and the problem of free agency, which is closely related to externality, is also handled in immunization. Later, it was emphasized that negative externality as well as positive externality of immunization. In addition, the issue of free-ridership, which is closely related to externality, has been addressed in the context of immunization. Then, it is mentioned how subsidies, which are one of the methods of internalizing of collectivized externalities, can be used in immunization. It is thought that this study will contribute to the literature because there is no study in Turkish literature that combines the concepts of immunization and externality.