As described in Chapter 3, the committee recognized that the risk of an adverse effect of a vaccine can be influenced by host factors, some known and others not yet understood. Where the committee thought the evidence—whether from epidemiologic analyses or from the clinical studies—regarding risks to subpopulations was informative, evidence-based, and biologically sound, it made separate conclusions. For example, the risk of invasive disease following varicella vaccine, a live virus vaccine, is likely much higher in immunocompromised persons than in persons who are immunocompetent. Other subpopulation analyses in the report include age and sex for some specific adverse events.

In their consideration of several adverse events, the committee concluded that the mechanism of injury was likely unrelated to the specific antigenic or other components of the vaccine. The committee concluded that the exposure of concern is not the injected vaccine, rather the injection of the vaccine. The adverse events include syncope, complex regional pain syndrome, and deltoid bursitis. These are covered in Chapter 12.


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