The 1993 Pertussis Epidemic in Cincinnati

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Abstract
Christie and colleagues (July 7 issue)1 document the effect of the 1993 pertussis outbreak in Cincinnati and suggest that the proportion of cases in fully vaccinated children provides evidence of “the failure of the whole-cell pertussis vaccine.” However, their data prove only that pertussis vaccine is not 100 percent effective and that outbreaks of disease can occur in highly vaccinated populations. In outbreaks of any disease that can be prevented by vaccination, the proportion of cases in people who have been vaccinated will depend on both the efficacy of the vaccine and the vaccination coverage of the exposed population. For example, if 1000 children receive a vaccine that is 90 percent effective, 100 children will be fully vaccinated yet still susceptible. Should an outbreak of disease occur in such a population, 100 percent of the children will have a history of vaccination, yet 90 percent of the vaccinated children will be protected from disease.