Figure 1. . . Timeline of pertussis vaccine introduction in the United States and appearance of alleles within the Bordetella pertussis population, 1935–2009. The 8 periods used in this study are indicated at bottom; numbers below indicate number of selected strains during that period (N = 661). wP, whole-cell pertussis vaccine; MLVA 27, multilocus variable number tandem repeat analysis type 27; aP, acellular pertussis vaccine; Tdap, tetanus-diphtheria-aP.
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