ABSTRACT
Many factors influence the selection of vaccines that are administered to infants and toddlers in a particular country and the immunization schedule that is followed. These include epidemiological (e.g., past and present burden and prevalence of specific infectious diseases), economic (e.g., relative cost of vaccines, including various combination vaccines), sociological (e.g., the public perception of the risk and severity of specific infectious diseases and of the safety and efficacy of specific vaccines), logistics (e.g., population dispersion, mobility, and access to health services), immunological (e.g., differences in the immune responses to specific vaccine antigens depending on age and spacing of doses), and historical factors. For infant immunizations, the national schedule for diphtheria/pertussis/tetanus (DPT) vaccine largely drives the overall infant immunization schedule. Interestingly, in most of the developing world, the Expanded Program on Immunization (as described by Bruce Aylward et al. in the chapter entitled “Reaching Every Child-Achieving Equity in Global Immunization”) administers infant DPT immunizations according to only two main schedules. In the least developed countries, where the risk of pertussis in early life is a particular danger, doses of DPT are administered at 6, 10, and 14 weeks of age. Because of economic and logistical constraints in these countries, a booster immunization in the second year of life is generally not practiced. In contrast, in many newly developing and middle-income countries, where the incidence of pertussis is lower and immunization coverage is higher, such as in Latin America and parts of Asia, infant DPT immunizations are typically administered at 2, 4, and 6 months of age; these countries usually also give boosters in the second year of life (often at 18 months of age). Thus, among the
nonindustrialized countries there exists an extraordinary degree of harmonization of pediatric immunization schedules with most countries following one of two schedules, although the array of specific vaccines administered at these times varies greatly (largely driven by economic considerations).
