Introduction: vaccination against smallpox in the posteradication era.

The following introduction describes the context in which the national smallpox vaccination program was implemented and highlights the significance of the key policy, programmatic, or scientific challenges, observations, and lessons learned that are presented in the articles that follow within this supplement to Clinical Infectious Diseases. Although the execution of this national program posed multiple complex and varied challenges, the focus of this supplement is on vaccine-associated adverse events and vaccine safety.