A large outbreak of pertussis occurred in California in 2010. This investigation indicates that waning immunity in children after the fifth recommended diphtheria–tetanus–acellular pertussis vaccination may have contributed substantially to the outbreak. Pertussis is a worldwide, cyclic infection. Before widespread vaccine coverage, up to 270,000 cases of pertussis were diagnosed in the United States annually, with as many as 10,000 deaths per year, predominantly among infants. 1 Pertussis vaccines prepared from whole Bordetella pertussis organisms were available from the 1940s through the 1990s, protecting infants who were 2 months of age or older. 1 Whole-cell pertussis vaccines, when administered as part of a combined diphtheria, tetanus toxoids, and pertussis vaccine, were effective, but they were associated with adverse effects 2 ; this led to the development of the diphtheria–tetanus–acellular pertussis (DTaP) vaccine. 3 Beginning in the . . .