Background. Meningococcal conjugate vaccines against serogroups A, C, W, and Y (MenACWY) are recommended for routine use in adolescents aged 11–18 years. The impact of these vaccines on the meningococcal population structure in the United States have yet to be evaluated.
Methods. Meningococcal isolates recovered during 2006–2010 (ie, after introduction of MenACWY) collected through Active Bacterial Core surveillance (ABCs) were characterized; serogroup distribution and molecular features of these isolates were compared to previously published data on ABCs isolates recovered from 2000 to 2005 (ie, before introduction of MenACWY). P values were generated using χ2 statistics and exact methods.
Results. There was a significant change (P < .05) in serogroup distribution among all age groups between the 2 periods. A small proportion of isolates showed evidence of capsular switching in both periods. Between the 2 periods, significant changes were observed in the distribution of porin A, ferric enterobactin transport, and strain genotypes among vaccine and nonvaccine serogroups.
Conclusions. The population structure of US meningococcal isolates is dynamic; some changes occurred over time, but the basic structure remained. Vaccine-induced serogroup replacement was not observed, although a small proportion of isolates had undergone capsule switching, possibly driven by non–vaccine-mediated selection. Changes in the distribution of molecular features are likely due to horizontal gene transfer and changes in serogroup distribution.