the varicella vaccine strain, usually in immunodeficient persons. Varicella vaccine is a live virus vaccine that is contraindicated in people with known, severe immunodeficiency, including severe combined immunodeficiency, other congenital immunodeficiencies, and immunodeficiency arising from long-term immunosuppressive therapy or from chemotherapy for hematologic or solid tumors. The evidence for the causal relationships for adverse events from infection by the vaccine virus came from case reports, so there was no cohort or background population to allow calculation of a rate, even among the population of people who have demonstrated immunodeficiencies.

Although the lifetime rate of shingles in the population has been estimated (9–10 percent for those under 45 years of age, 22–32 percent in older persons) (Chapman et al., 2003), the rate of shingles and other infection-related adverse events associated specifically with the varicella vaccine virus are not known for several reasons. First, while the rate of shingles can be estimated (see Chapman, above), in most cases the virus is not characterized, meaning no test is done to determine whether the virus is wild or vaccine type. Second, while the Food and Drug Administration (FDA) and the Centers for Disease Control and Prevention (CDC) oversee a large database of reports, the Vaccine Adverse Event Reporting System (VAERS), those reports are often incomplete and do not always have the information that would document the vaccine strain or the presence or absence of immunodeficiency. However, it appears likely to the committee that the risk of vaccine-strain varicella infection and subsequent serious disease to persons demonstrated to be immunocompetent is exceedingly low, while the risk to those with severe immunodeficiency is real, which is what the CDC and FDA have concluded by deciding that varicella vaccination is contraindicated in such persons. And, of course, immunocompromised individuals benefit greatly from a high level of immunity to varicella within the community.

Anaphylaxis: Although it is also difficult to estimate rates for very rare conditions, the committee concluded that evidence supports the association of anaphylaxis with certain vaccines in certain circumstances, but the number of events related to each specific vaccine is not known. Rates can be estimated from surveillance studies, but often specific details are missing, and each case cannot be linked with certainty to vaccine. For example, Bohlke et al. (2003) (an uncontrolled study from the Vaccine Safety Datalink [VSD] network) reported three cases of anaphylaxis after administration of 848,945 doses of MMR vaccine. However, two of those children received vaccines in addition to MMR, and the confidence interval for the calculated rate per million doses was very wide (rate per million doses = 3.5, 95% CI, 0.7–10.3). Lastly, regarding this example of a rare condition, not only is the number of true anaphylactic reactions to vaccines not known, but also

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