title: Stability of the pneumococcal population structure in Massachusetts as PCV13 was introduced.
abstract: BACKGROUND
  The success of 7-valent pneumococcal conjugate vaccination (PCV-7) introduced to the US childhood immunization schedule in 2000 was partially offset by increases in invasive pneumococcal disease (IPD) and pneumococcal carriage due to non-vaccine serotypes, in particular 19A, in the years that followed. A 13-valent conjugate vaccine (PCV-13) was introduced in 2010. As part of an ongoing study of the response of the Massachusetts pneumococcal population to conjugate vaccination, we report the findings from the samples collected in 2011, as PCV-13 was introduced.
  
  
  METHODS
  We used multilocus sequence typing (MLST) to analyze 367 pneumococcal isolates carried by Massachusetts children (aged 3 months-7 years) collected during the winter of 2010-11 and used eBURST software to compare the pneumococcal population structure with that found in previous years.
  
  
  RESULTS
  One hundred and four distinct sequence types (STs) were found, including 24 that had not been previously recorded. Comparison with a similar sample collected in 2009 revealed no significant overall difference in the ST composition (p = 0.39, classification index). However, we describe clonal dynamics within the important replacement serotypes 19A, 15B/C, and 6C, and clonal expansion of ST 433 and ST 432, which are respectively serotype 22F and 21 clones.
  
  
  CONCLUSIONS
  While little overall change in serotypes or STs was evident, multiple changes in the frequency of individual STs and or serotypes may plausibly be ascribed to the introduction of PCV-13. This 2011 sample documents the initial impact of PCV-13 and will be important for comparison with future studies of the evolution of the pneumococcal population in Massachusetts.
authors.affiliation: Department of Epidemiology, Harvard School of Public Health, 677 Huntington Avenue, Boston, MA, USA. qic716@mail.harvard.edu.
authors.forename: Qiuzhi
authors.initials: Q
authors.lastname: Chang
authors.affiliation: Department of Epidemiology, Harvard School of Public Health, 677 Huntington Avenue, Boston, MA, USA. aesteven21@gmail.com.
authors.forename: Abbie E
authors.initials: AE
authors.lastname: Stevenson
authors.affiliation: Department of Epidemiology, Harvard School of Public Health, 677 Huntington Avenue, Boston, MA, USA. nickjcroucher@gmail.com.|Department of Infectious Disease Epidemiology, Imperial College London, London, W2 1PG, UK. nickjcroucher@gmail.com.
authors.forename: Nicholas J
authors.initials: NJ
authors.lastname: Croucher
authors.affiliation: Center for Child Health Care Studies, Department of Population Medicine, Harvard Pilgrim Health Care Institute and Harvard Medical School, Boston, MA, USA. glee.hms@gmail.com.|Division of Infectious Diseases, Boston Children's Hospital, Boston, MA, USA. glee.hms@gmail.com.
authors.forename: Grace M
authors.initials: GM
authors.lastname: Lee
authors.affiliation: Maxwell Finland Laboratories, Boston University School of Medicine, Boston, MA, USA. spelton@bu.edu.
authors.forename: Stephen I
authors.initials: SI
authors.lastname: Pelton
authors.affiliation: Department of Epidemiology, Harvard School of Public Health, 677 Huntington Avenue, Boston, MA, USA. MLIPSITC@hsph.harvard.edu.
authors.forename: Marc
authors.initials: M
authors.lastname: Lipsitch
authors.affiliation: Center for Child Health Care Studies, Department of Population Medicine, Harvard Pilgrim Health Care Institute and Harvard Medical School, Boston, MA, USA. Jonathan.Finkelstein@childrens.harvard.edu.|Division of General Pediatrics, Boston Children's Hospital, Boston, MA, USA. Jonathan.Finkelstein@childrens.harvard.edu.
authors.forename: Jonathan A
authors.initials: JA
authors.lastname: Finkelstein
authors.affiliation: Department of Epidemiology, Harvard School of Public Health, 677 Huntington Avenue, Boston, MA, USA. whanage@hsph.harvard.edu.
authors.forename: William P
authors.initials: WP
authors.lastname: Hanage
mesh_terms: D002648:Child; D002675:Child, Preschool; D005260:Female; D006801:Humans; D007223:Infant; D008297:Male; D008404:Massachusetts; D058885:Multilocus Sequence Typing; D011008:Pneumococcal Infections / Q000382:microbiology* / Q000517:prevention & control; D022242:Pneumococcal Vaccines; D012703:Serotyping; D013296:Streptococcus pneumoniae / Q000145:classification*; D018074:Vaccines, Conjugate
chemical_list: C538862:13-valent pneumococcal vaccine; D022242:Pneumococcal Vaccines; D018074:Vaccines, Conjugate