Children may receive as many as 24 injections by 2 years of age and up to 5 injections in a single visit (see Appendix A). Immunization schedules vary around the world, however, with the variability being due in part to the different patterns of disease that exist globally (Lopalco et al., 2009; WHO, 2012). Additionally, levels of antigens and immunization timing and number differ. Some countries also have differing approaches to postmarketing surveillance systems, as will be described in Chapter 3.

Although the number of vaccinations recommended is greater than ever before, the vaccines used in the current immunization schedule actually have fewer antigens (inactivated or dead viruses and bacteria, altered bacterial toxins, or altered bacterial toxins that cause disease and infection) because of developments in vaccine technology (Offit et al., 2002). For example, the vaccines to prevent whooping cough used before 1991 contained 3,000 different potentially antigenic proteins (IOM, 2002). From 1980 to 2000, the immunization schedule’s total number of antigens decreased by approximately 96 percent (from 3,041 to 123-126) (Offit et al., 2002).

Ever since vaccines were introduced in the 18th century, questions and concerns about their safety have been voiced. However, the protection against feared, deadly diseases that vaccines offer encourages the majority of health care professionals and laypeople to support immunization (Stern and Markel, 2005). Although research on the adverse effects of individual vaccines is robust and a required part of the approval process by ACIP, questions about the safety of the entire recommended immunization schedule for children persist. Moreover, how safety is interpreted varies according to the severity of an adverse event and the benefit of the vaccine. For example, some might believe that one serious adverse event that occurs once in 1 million doses is “safe enough” compared with the benefit of prevention of serious disease, whereas others may consider that risk unacceptably high.

As the number of recommended vaccines has increased in recent years, some parents and advocacy groups have expressed the concern that the immunization schedule is too crowded and complex because of the increasing number of vaccines administered during the first 2 years of a child’s life (Offit et al., 2002). In addition to the complexity of vaccine delivery, some people have raised questions about the potential for adverse health outcomes as a consequence of the simultaneous or sequential administration of childhood vaccines (Gregson and Edelman, 2003). Even though the current childhood immunization schedule offers flexibility for administration of recommended vaccines (see Appendix A), some parents elect not to follow the recommended schedule (Dempsey et al., 2011).

Analysis of current U.S. data shows that the vaccination rate among children entering kindergarten exceeds 90 percent for most recommended vaccines (CDC, 2012b). However, increases in the prevalence of delay or refusal of recommended vaccines have contributed to the emergence of

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