more susceptible to pathogens, many of which may lead to serious or fatal infections. Therefore, to be effective, a vaccine should be administered early enough to protect the infant or child against preventable diseases.

The age range for which a childhood vaccine is developed and recommended as part of the immunization schedule takes into account the age at which the immune system can tolerate vaccine components, potential interference with the immune response from maternal antibodies, and the age at which a child is most at risk for disease transmission and mortality. ACIP recommends vaccines “for members of the youngest age group at risk for experiencing the disease for which efficacy and safety have been demonstrated,” and its recommendations are based on the best evidence available (CDC, 2011a, p. 4).

IMMUNIZATION AT THE POPULATION LEVEL

For immunizations to adequately protect individuals and the individuals in the communities in which they live against outbreaks of vaccine-preventable diseases, a high proportion of vaccinated individuals needs to be maintained in the general population. The success of vaccination to preserve low levels of disease incidence depends on the population level of “community immunity,” also commonly known as herd immunity, which refers to the immunity of a group that is afforded when a high proportion of individuals are not susceptible to infection. Community immunity is maintained by vaccination against communicable diseases, and this concept is expertly discussed in other sources (Fine, 1993; Fine et al., 2011).

It is possible to quantify the fraction of the population that needs to be protected to prevent disease spread on the basis of the epidemiological traits of the pathogen in question (such as its transmissibility and duration of infectivity). The calculation requires an understanding of the so-called basic reproduction ratio, or R0, which quantifies the maximum transmission potential of an infectious disease. It is strictly defined as the number of secondary cases generated by a typical primary case in a fully susceptible population. If R0 is >1, then the pathogen is predicted to transmit to more than one other person and successfully invade the population. For the major childhood infectious diseases, such as measles, mumps, rubella, chickenpox, and polio, a variety of methods have been devised to estimate R0 from longitudinal incidence reports, outbreak data, and age-stratified serology (Anderson and May, 1982, 1992; Becker, 1989; Keeling and Rohani, 2008).

The quantity R0 has been used to guide vaccination policy with recognition that it is defined when the entire population is susceptible to the pathogen. That is, the number of susceptible individuals (S) is equal to the population size (N). To determine the size to which the pool of susceptible individuals needs to be reduced (via immunization) to control the infection,



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