Figure 4. . . Frequency (by proportion of all isolates tested) of predominant multilocus variable number tandem repeat analysis (MLVA) types within the Bordetella pertussis population, United States, 1935–2009. MLVA 10 was dominant in period 2 (1946–1969) but decreased through periods 3 (1970–1990) and 4 (1991–1996) while MLVA 18, 27, and 29 emerged. MLVA 27 increased in proportion during period 4 and dominated the population for the rest of the study period; however, the proportion of MLVA 27 has been decreasing since period 6 (2000–2002), allowing for the emergence of other types.