During the 2010 pertussis epidemic in California, there was considerable concern in the press and in public health communications about the possible contribution of vaccine failures to the problem.1,2 In this commentary, I examine why pertussis vaccines fail and Table 1 lists 8 possible reasons.

The first reason, and perhaps the most important one, is that our estimates of vaccine efficacy have been inflated because of case definition.3,–11 At the time of the pediatric diphtheria and tetanus toxoids and acellular pertussis (DTaP) vaccine efficacy trials in the early 1990s, it was hoped that a universal case definition could be developed so that the results of the various trials could be compared. To this end, the World Health Organization (WHO) case definition was developed.3 The primary case definition required laboratory confirmation and ≥21 days of paroxysmal cough. I was a member of the...

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