Before approval by the Food and Drug Administration (FDA), vaccines are evaluated for efficacy and safety using large Phase III randomized controlled trials. For childhood vaccines, the number of children enrolled in these trials is typically in the thousands. That is sufficient to detect common but not rare adverse events. For the latter, there exist several postmarketing vaccine safety surveillance systems using observational data on children who receive the vaccines as part of their general care. In the United States, these include the Vaccine Adverse Event Reporting System (VAERS), the Vaccine Safety Datalink (VSD), and the Clinical Immunization Safety Assessment Network, all sponsored by the Centers for Disease Control and Prevention (CDC), as well as the Post-Licensure Rapid Immunization Safety Monitoring System (PRISM), which is part of the FDA-sponsored Mini-Sentinel Initiative. Internationally, there are other important vaccine safety surveillance systems such as the Epidemiology Vaccine Research Program at the National Institute for Health Data and Disease Control in Denmark; the Vaccine Adverse Event Surveillance and Communication (VAESCO) Network, coordinated by the European Center for Disease Control; the World Health Organization (WHO) Collaborating Centre for International Drug Monitoring; and the Immunization Division at the Communicable Disease Surveillance Centre in England. All these vaccine safety systems have proven to be very useful and important. They have detected unsuspected adverse events leading to revisions in vaccine recommendations and, in other cases, established the safety of vaccines for which important safety concerns existed. Throughout their existence, there has been continuous and rapid development with respect to the types of questions studied and the epidemiological and statistical methods used. For example, for every new childhood vaccine approved by the FDA, VSD now conducts near real-time safety surveillance using weekly data feeds from electronic health records (Lieu et al., 2007; Yih et al., 2011). The credit for these continuously improved vaccine safety surveillance systems goes to the devoted scientists that are building the systems and using them for many important studies, to the government agencies supporting this work, and to the vaccine safety advocacy groups that are the key public voice for improved and expanded vaccine safety surveillance.

Most postmarketing studies evaluate the general question as to whether or not a vaccine causes an adverse event. Very few postmarketing studies have evaluated whether the risk of adverse events depends on the scheduling of the vaccines. For example, few postmarketing studies have evaluated whether the risk of adverse events depends on the age at which a vaccine is given, on the relative timing of two different vaccines, or on a combined cumulative effect generated by the timing of dozens of different vaccines.

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