fund and support the Vaccine Safety Datalink project to study the safety of the recommended immunization schedule. Furthermore, HHS should consider expanding the collaboration with new health plan members and enhancing the data to improve its utility and generalizability.

METHODS TO MONITOR COMMUNITY IMMUNITY AND MEASURE POPULATION-LEVEL IMPACTS OF STUDIES OF THE IMMUNIZATION SCHEDULE

If large numbers of children avoided immunization, community immunity would be eroded and this protective effect would disappear for those who are not or who cannot be fully vaccinated. Thus, any analysis of vaccine safety data needs to consider the community immunity aspect of the milieu in which the study is conducted. Such complications would affect both clinical trials and observational studies.

Consideration of Population Impacts of Alternative Schedules

Attempts to quantify the relative safety of contrasting immunization schedules need to take into account at least two separate health outcomes: (1) adverse events related to the administration of specific vaccines and the overall immunization schedule, and (2) the respective impacts of alternative schedules on the circulation of vaccine-preventable diseases and the consequent adverse outcomes associated with infection. Secondary effects (such as longer waiting times and the greater cost of care if more visits are needed for immunization) and potential medical errors in provider offices accustomed to the routine schedule would also have to be measured.

Previously, high-profile analyses have focused on calculation of the number of serious reactions either per vaccine or over the immunization schedule compared with the per child risk of hospitalization associated with vaccine-preventable diseases (Sears, 2011). Although such analyses are intuitively appealing, they overlook the intimate association between immunization and age-specific disease incidence. Specifically, any shifts in the immunization schedule that lead to a net increase in the time spent vulnerable to these diseases will inevitably increase the circulation of these pathogens. The population-level impacts of such an outcome will be a simultaneous rise in the incidence of the affected infectious diseases and a reduction in the age at which they are contracted. Thus, not only is the risk of exposure to vaccine-preventable diseases increased but so is the likely severity of infection, which may be most acute in younger children (Heiniger et al., 1997).

A clear manifestation of the dual impact of immunization on the incidence and age distribution of vaccine-preventable diseases has been documented in Sweden, where the pertussis vaccine was removed from the



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