Content area
Full text
Since the inception of vaccination, it has been recognized that adverse events following immunization (AEFIs) will occur. The frequency of AEFIs is directly related to the number of vaccine doses administered. AEFIs can be causally related to the inherent properties of the vaccine, linked to errors in the administration, quality, storage and transport of the vaccine (programmatic errors), but it must be recognized that when large populations are vaccinated, some serious events that occur rarely with or without vaccination will be observed coincidentally following vaccination. Thus, investigating causality of AEFIs, particularly those that are most serious, is challenging.
The clearest and most reliable way to determine whether an adverse event is causally related to vaccination is by comparing rates of the event in a vaccinated and non-vaccinated group in a randomized clinical trial. Such trials, however, can never be large enough to assess very rare events, and postmarketing surveillance systems are required to identify events potentially related to vaccination. Postmarketing surveillance capability is improving; more countries now have AEFI monitoring systems, and more importance is attached to the reporting of suspected links between vaccination and adverse events. These systems have been successful in bringing to light serious AEFIs after vaccines have been marketed. A recent example is intussusception after administration of reassortant rhesus rotavirus vaccine.
Assessments of whether a given vaccine causes a particular adverse reaction vary from the casual observation to the carefully controlled study. The majority of individuals are not trained in interpreting such studies and are unlikely to understand the enormous difference in significance between these two extremes. Nonetheless, the public frequently forms a decision about a vaccine's safety based on the information available to them - often a report based on unscientific observations or analyses that fail to stand the scrutiny of rigorous scientific investigation.
Certain reports of AEFIs published in the medical literature over the past few years have resulted in controversy. The studies on which these reports are based, while generating provocative hypotheses, have generally not fulfilled the criteria that would be needed to be able to draw conclusions about vaccine safety with any degree of certainty. Yet these reports have had a major influence on public debate and opinion-making. When this debate spills over to the political arena,...