Figure 2. . . . Proportion of each meningococcal serogroup among all isolates tested (1964–1984) or all cases (2006–2010), United States. Years of introduction of vaccine types are indicated by arrows. Unknown during 1964–1980 indicates isolates from a serogroup other than A/B/C/W135/Y or an unknown serogroup; during 1981–1984 indicates isolates that were not B, C, or Y; and during 2006–2010 indicates that no specimen is available and group is unknown. No data were available for 1985–2005. Data for 1964–1984 are from Brundage et al. (1) and Brundage and Zollinger (2). C, Neisseria meningitidis serogroup C; A, N. meningitidis serogroup A; quad, quadravalent (N. meningitidis serogroup s A, C, W-135, and Y).