the “denominator” of persons susceptible to anaphylaxis (rather than the general population of persons to be vaccinated) is unknown.

Anaphylactic reactions to several vaccines are likely caused by the presence of components introduced during manufacturing, such as egg protein, milk protein, or gelatin. When a specific inciting component of the vaccine has been identified and the manufacturers find ways to remove or drastically reduce the amount of the reactive antigen (e.g., egg protein in influenza vaccine), the number of reports of anaphylaxis in spontaneous reporting systems has decreased. It appears likely to the committee that the risk of anaphylaxis caused by vaccines is exceedingly low in the general population. The risk is obviously higher in people with known and demonstrably severe allergies to certain vaccine components, such as eggs or gelatin.

An affirmative finding for causality was determined for a very mild condition (oculorespiratory syndrome) subsequent to certain influenza vaccines used only in two seasons in Canada. The committee made no attempt to determine the rate of this condition.

Finally, the committee determined that evidence supported an association with what the committee considered to be injection-related events: deltoid bursitis and syncope. These injection-related events are known to be caused by many things other than vaccine administration and are likely often unreported. Estimates of the rates caused by vaccination are similarly not available, as population-based studies have not been conducted.

The seriousness of any particular adverse effect is a complex question, taking into account such factors as the degree and duration of disability and the type of health care needed as a result, recognizing that any individual who experiences an adverse effect may regard it as serious. All of these considerations have social and ethical components as well. Deeming this calculus to be too complex to define with particularity, the committee elected to defer to common understanding within the health care community for assessment of the seriousness of any particular adverse effect.

An issue that is likely to be of concern to some readers regards the very stringent approach our committee has taken. For the majority of adverse events the committee was asked to examine, the committee concludes that the evidence is inadequate to accept or reject a causal relationship. Some might interpret that to mean either of the following statements:

  • Because the committee did not find convincing evidence that the vaccine does cause the adverse event, the vaccine is safe.
  • Because the committee did not find convincing evidence that the vaccine does not cause the adverse event, the vaccine is unsafe.

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