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Vaccines are widely recognized as one of the greatest public health successes of the last century, significantly reducing morbidity and mortality from a variety of bacteria and viruses. Diseases that were once the cause of many outbreaks, common causes of loss of health and life, are now rarely seen, because they have been prevented by vaccines. However, vaccines can in rare cases themselves cause illness. A rare potential for harm can loom large when people no longer experience or fear the targeted disease. In this regard, the public opinion of vaccines can be a victim of their success. The Institute of Medicine (IOM) was charged by Congress when it enacted the National Childhood Vaccine Injury Act in 1986 with reviewing the literature regarding the adverse events associated with vaccines covered by the program, a charge which the IOM has addressed 11 times in the past 25 years. Following in this tradition, the task of this committee was to assess dispassionately the scientific evidence about whether eight different vaccines cause adverse events (AE), a total of 158 vaccine-AE pairs, the largest study undertaken to date, and the first comprehensive review since 1994.
The committee had a herculean task, requiring long and thoughtful discussions of our approach to analyzing the studies culled from more than 12,000 peer-reviewed articles in order to reach our conclusions, which are spelled out in the chapters that follow. In the process, we learned some lessons that may be of value for future efforts to evaluate vaccine safety. One is that some issues simply cannot be resolved with currently available epidemiologic data, excellent as some of the collections and studies are. Particularly for rare events, we look to the day when electronic medical records truly are universal and when society reaches a broad-based consensus about how these records may be used to detect very rare adverse events from vaccines as well as other drugs and medical interventions. Even then, challenges will remain. Some adverse events caused by vaccines are also caused by the natural infection. These effects often cannot be detected by epidemiologic methods, which typically cannot distinguish between the adverse events that are caused by the vaccine itself and the decrease in adverse events due to the decreased rate of natural infection. In addition, even very large epidemiologic studies may not detect or rule out rare events. Subgroup analysis or more focused epidemiologic studies, informed by as yet incomplete knowledge of the biologic mechanisms of vaccine-induced injury, may be required.
Examining mechanistic evidence to assess causation is also challenging. Many of the case reports the committee reviewed simply cited a temporal relation between vaccine administration and an adverse event. Association, however, does not equal causation. More is required. The proof can be relatively straightforward, as when vaccine-specific virus is recovered from the cerebrospinal fluid of a patient who develops viral meningitis a few weeks after receiving the vaccine. Alleged adverse effects that appear to be immune mediated, as many of them are, are more challenging, in part because the biology is not completely understood. One potentially useful line of inquiry as science advances is to assess whether the vaccine recipient who suffers harm had a preexisting susceptibility to that particular adverse event as such studies may provide insight into the mechanisms by which such events occur. The committee is aware of the work funded by the Centers for Disease Control and Prevention (CDC) to study such individuals and looks forward to their findings. Most individuals, for example, who develop invasive infection from live vaccine viruses have demonstrated immunodeficiencies. Our work was also complicated by the wide variation in the case reports regarding what other tests had been done to rule out other potential causes. To improve the utility of these reports, periodically convening a group of experts to suggest guidelines, based on the best available science, for providing mechanistic evidence that a particular adverse event was caused by a vaccine may be useful. These guidelines could be made available on the Web, and perhaps more important, shared with clinicians who report cases to the Vaccine Adverse Event Reporting System so their reports can be as complete and useful as possible.
The value of dialogue between both epidemiologic and mechanisms approaches cannot be overstated. Epidemiologic studies can identify particular at-risk groups, who can then be examined with more in depth testing to explore predisposing factors. The findings of such studies can then inform more focused epidemiologic research as well as efforts to reduce risks. These conversations between different types of research can be difficult, but the results are worth it.
Although the committee is optimistic that more can and will be known about vaccine safety in the future, the limitations of the currently available peer-reviewed data meant that, more often not, we did not have sufficient scientific information to conclude whether a particular vaccine caused a specific rare adverse event. Where the data were inadequate to reach a scientifically defensible conclusion about causation, the committee specifically chose not to say which way the evidence “leaned,” reasoning that such indications would violate our analytic framework. Some readers doubtless will be disappointed by this level of rigor. The committee particularly counsels readers not to interpret a conclusion of inadequate data to accept or reject causation as evidence either that causation is either present or absent. Inadequate data to accept or reject causation means just that—inadequate. It is also important to recognize what our task was not. We were not charged with assessing the benefits of vaccines, with weighing benefits and costs, or with deciding how, when, and to whom vaccines should be administered. The committee was not charged with making vaccine policy. We did receive calls to stride into this contentious debate, but others, such as the Food and Drug Administration and the CDC, are tasked with formulating recommendations for use that balance the risk of vaccines with the benefits, with studying the safety of the vaccines during pre-release trials, and monitoring them closely once the vaccine is in use in the population.
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