Increase in Serotype 6C Pneumococcal Carriage, United Kingdom

To the Editor: Streptococcus pneumoniae is a major human pathogen. In 2007, Park et al. identifi ed a novel serotype, 6C (1), which emerged from serotype 6A. A study of children in the Netherlands who had not previously received a pneumococcal vaccine found low prevalence of this newly identifi ed serotype before the heptavalent pneumococcal conjugate vaccine Prevnar/Prevenar (PCV7) (Wyeth, Taplow, UK) was introduced (2). Studies have shown cross-protection between vaccine serotype 6B and vaccine-related serotype 6A. However , PCV7 elicits no cross-protection against serotype 6C. The potential exists for the emergence of nonvaccine serotypes or novel clones. These serotypes and clones may be better adapted to colonize the na-sopharynx, evade the human immune response, and cause disease. A recent study showed an increase in prevalence of serotype 6C pneumococci in children and a corresponding decrease in serotype 6A after introduction of PCV7 (3). We studied the underlying genetic basis for expansion of serotype 6C. Initial data from an ongoing study of pneumococcal carriage are presented. This study was reviewed and approved by the Southampton and South West Hampshire Research Ethics Committee (B) (reference 06/Q1704/105). A total of 697 nasopharyngeal swab specimens were collected from unse-lected (not selected by a method) children <4 years of age in the pediatric outpatient department of a large teaching hospital in the United Kingdom. Samples were obtained during October 2006–March 2007, during implementation of PCV7 in the infant immunization schedule of the United Kingdom. During October 2007–March 2008, a total of 202 pneumococci were isolated. All pneumococci were characterized by serotype and genotype. In the fi rst year of this study, we identifi ed 3 (3.1%) serotype 6C pneumococci belonging to 3 se-and ST 1692 shared a common clonal complex. Only ST 65 was shared between serotype 6C and serotype 6A. In the second year, we identifi ed 14 (13.6%) serotype 6C pneumococci belonging to 6 STs (online Appendix Figure). Two of these STs, of the same ST, were from siblings. Three of them (ST1692 [n = 8], ST1714 [n = 2], and ST395 [n = 1]) were members of a common clonal complex with a predicted founder of ST395. Each of the remaining 3 STs (ST398, ST1862, and ST3460) was isolated only once. One serotype 6A isolate of ST1692 was also observed. No serotype 6C ST65 was observed in the second year. We isolated more serotype 6C pneumococci in year 2 than in …