Publisher Summary This chapter examines the comparative and observational studies of vaccines. The surveillance of adverse events following immunization is elaborated. It is found that the development of type 1 diabetes in genetically predisposed children was not significantly associated with immunization. The association of anthrax immunization with arthritic, immunological, and gastrointestinal adverse reactions has been evaluated. The incidence of adverse reactions reported after anthrax vaccine was higher for every reaction analyzed compared with the adult vaccine control groups. It is suggested that the rate of Guillain–Barré syndrome based on the number of cases reported within 6 weeks of administration of MCV4 is similar to what might have been expected to occur by chance alone. An 11-valent pneumococcal conjugate vaccine was given to 117 Finnish and 135 Israeli infants at ages 2, 4, 6, and 12 months together with other vaccines used in the respective national immunization programs. The subacute sclerosing panencephaliti and measles immunization are also elaborated in this chapter.