Background: Multilocus sequence typing (MLST) is commonly used to understand the genetic background of invasive pneumococcal disease (IPD) isolates. This study was conducted to identify serotype and genetic change among IPD isolates in Canadian children following vaccine use.
Methods: Clinical isolates collected from children ≤5 years old of Ontario, Canada with IPD during 2007-2012 were characterized with serotyping, multilocus sequence typing and antimicrobial susceptibility testing.
Results: One year after 13-valent pneumococcal conjugate vaccine (PCV13) implementation, a decline in 19A and 7F was observed in 2012, coincident with the rise of serogroup 15 and 22F. Clonal complex (CC) 199, CC320 and CC695 are 3 major CCs in 19A (74%). From 2007 to 2012, clonal shift was detected in the 19A population as CC320 and CC199 declined, whereas CC695 rose to a majority. Genetically, serogroup 15 was composed of 2 CCs and 7 sequence types (STs), making it more diverse than serotypes 3, 7F and 22F. Interestingly, 60% of 15C isolates were a novel ST, suggesting high single nucleotide polymorphism frequency in house-keeping genes of 15C. Several newly appeared STs found in 19A and 15 indicate the possibility of recent serotype switching events.
Conclusion: Genetic shift because of PCV13 impact may have resulted in the decline of 19A in IPD. Recent rise of serogroup 15 infections in children could be because of its selective advantage conferred by genetic diversity, frequent recombination in the population plus drug resistance potential related to CC63 genotype. Close monitoring of serotype replacement and genetic change in IPD among children post-PCV13 is warranted.