Background. A putative Streptococcus pneumoniae serotype, 6D, resulting from the introduction of wciN b into serotype 6B has been proposed. Methods. We studied 98 unique serogroup 6 isolates from Fijian children, two-thirds of whom had received at least 1 dose of 7-valent pneumococcal conjugate vaccine, and 51 invasive isolates from Australian children. We used a polymerase chain reaction (PCR) system that targets both wciN b and the single-nucleotide polymorphism that differentiates serotypes

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... A and 6B-wciP584g (6A) and wciP584a (6B). Results. Two (9%) of 22 Australian isolates and 24 (38%) of 64 Fijian isolates previously identified as 6A by the Quellung reaction and wciP584g PCR contained wciN b and were designated as 6C; 14 (41%) of 34 Fijian isolates previously identified as 6B by the Quellung reaction and wciP584a PCR contained wciN b and were designated as the new putative serotype 6D. A significantly smaller proportion of children from whom serotype 6D was isolated (2/14 [14%]) had not received PCV-7, compared with the proportion of those from whom serotype 6B was isolated (11/20 [55%]) ( ). P ! .05 Conclusion. This is the first report of naturally occurring S. pneumoniae serotype 6D.