trolled epidemiologic studies of high methodologic quality and sufficient statistical power (sample size), failure to detect an association between a vaccine and a particular adverse event was judged as favoring rejection of a causal relation.

Even though the committee was not specifically charged with assessing the causal role of vaccines in individual cases, such assessments can be useful in evaluating Can It? causality. For many of the vaccine-adverse event associations under consideration, no epidemiologic studies have been reported, and individual case reports provide the only available evidence. As discussed above, if that evidence strongly suggests that the vaccine did cause the adverse event in one or more cases, then it is logical to conclude that it can cause the event.

In fact, many of the associations that the committee was charged with examining were first suggested because one or more cases of adverse events were found to occur following receipt of the vaccine. Some of these originated from case reports in the published medical literature; others originated from reports by physicians, nurses, parents, or vaccine recipients who observed the adverse event following exposure to the vaccine. The arousal of one's suspicions that a vaccine might be the cause of an adverse event that occurs within hours. days, or weeks following receipt of the vaccine is natural and understandable. But the mere fact that B follows A does not mean that A caused B; inferring causation solely on the basis of a proper temporal sequence is the logical fallacy of post hoc ergo propter hoc (literally, "after this, therefore because of this").

Many factors go into evaluating the causal relation between vaccine exposure and adverse events from individual case reports. Much of the literature in this area has come from postmarketing surveillance programs that monitor adverse drug reactions, such as those programs maintained by the U.S. Food and Drug Administration and comparable agencies in other countries (Venulet, 1982). Such passive, "spontaneous reporting" programs have been shown to have problems with both false-negative and false-positive results; that is, many of the reported cases are probably not caused by exposure to the drug or vaccine, whereas many drug- or vaccine-caused events go unreported (Faich, 1986; Péré, 1991; Tubert et al., 1992).

The information from case reports that is useful in assessing causality can be considered under the following seven headings (Kramer, 1981):

1. Previous general experience with the vaccine: How long has it been on the market? How many individuals have received it? How often have vaccine recipients experienced similar events? How often does the