of guidance on vaccine shortages. Although vaccine safety is rigorously assessed during prelicensing clinical trials, this postmarketing monitoring is important because the sample sizes in prelicensing clinical trials may not have been adequate to detect rare adverse events, the prelicensing study population may not have been monitored for long-term adverse events, and populations may not have been heterogeneous (Baggs et al., 2011; Chen et al., 2000). Consequently, postmarketing evaluation of vaccine safety is needed to assess rare, delayed, or unusual reactions and in general provides a fuller understanding of the safety of vaccines recommended in the immunization schedule (Chen et al., 1997).

Ongoing surveillance systems serve as the primary resource for information and research on postmarketing vaccine safety. The CDC Immunization Safety Office (ISO) maintains three major postmarketing surveillance systems: the Vaccine Adverse Event Reporting System (VAERS; jointly managed with FDA), the Vaccine Safety Datalink (VSD), and the Clinical Immunization Safety Assessment (CISA) Network. Most CDC immunization activities are located in the National Center for Immunization and Respiratory Diseases. Since 2005, the ISO was moved to the National Center for Emerging and Zoonotic Infectious Diseases as its mission is clearly distinct from other immunization programs within the agency. This organizational change ensures the separation at CDC between vaccine promotion and safety. In addition to the surveillance systems managed by CDC, FDA has established a supplementary mechanism for monitoring vaccine safety called the Sentinel Initiative.

Vaccine Adverse Event Reporting System

VAERS is a passive reporting surveillance system that is jointly managed by CDC and FDA and serves as a warning system for potential adverse events and side effects from a recommended vaccine that may not have been detectable in clinical trials (NVAC, 2011). Anyone, including parents and providers, may submit voluntary, spontaneous reports of adverse events observed after administration of licensed vaccines. Reports received by VAERS are analyzed and recorded for possible follow-up (CDC and FDA, 2012).

Although VAERS is useful for the early detection of signals of adverse events, the data obtained from the system have limitations. The reports received may not be fully documented, or the adverse event attributed to the vaccine may, in actuality, be a case not caused by the vaccine on the basis of background rates of clinical events. In addition, data on the number of doses of vaccine administered or number of vaccinated people do not exist and are thus not available for use as the denominator, so researchers cannot calculate what proportion of individuals were affected by an adverse event for comparison with the background rate of the event in the general



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