Figure 2. . . Minimum spanning trees depicting changes within the Bordetella pertussis population, United States, 1935–1996. Multilocus variable number tandem repeat analysis (MLVA) types are represented by circles and are scaled to member count within each panel; multilocus sequence typing (MLST) types are represented by color. A) Periods 1 and 2, 1935–1945 (n = 3) and 1946–1969 (n = 16), respectively. These 2 periods were combined for the generation of the tree. Period 1 (prevaccine era) strains are shown on the left, distantly related to period 2 strains (right side) from the early whole-cell pertussis vaccine (wP) era. B) Period 3, 1970–1990, n = 76. During this period, when wP was in use, a high degree of diversity was identified; 2 predominant MLST types differed by the ptxS1 allele. C) Period 4, 1991–1996, n = 86. During the transition from wP to acellular pertussis vaccine for the 4th and 5th dose of the childhood series, MLVA 27, ptxP3, and prn2 were dominant, and the fim3B allele emerged.