Streptococcus pneumoniae (SP) strains carried in the nasopharynx of healthy children reflect the circulating strains implicated in respiratory and invasive infections.
We prospectively analyzed the serotype distribution of SP nasopharyngeal isolates in healthy toddlers aged 21 to 78 months (median age, 63 months) attending day care centers during the winters of 2010–2012. All children were sampled once during the study period. Serotyping was performed by the Quellung reaction using the 12 pooled antisera Pneumotest panel and specific factor sera. We also studied the in vitro antimicrobial susceptibilities of the isolated pneumococci to several antibiotics by E-test, and for macrolide-resistant isolates of 2010 only, the resistance genes mef(A) and erm(B).
Among the 543 toddlers studied, 98.8% had received 1 or more doses of 7-valent pneumococcal conjugate vaccine (PCV7), whereas none was immunized with PCV10 or PCV13. Overall, 27.4% of the children tested were colonized with SP, whereas 22.8% and 52.3% of the isolates belonged to PCV7 and PCV13 serotypes, respectively. Overall, 25.5% of the isolates had intermediate resistance to penicillin (minimal inhibitory concentration, 0.12–1 μg/mL) and 3.4% were resistant to penicillin (minimal inhibitory concentration, 2 μg/mL). All isolates were susceptible to amoxicillin, whereas 28.2% were resistant to macrolides, with the mef(A) gene–mediated M phenotype being the most common in 2010, the year that macrolide resistance genes were studied (93.4% of macrolide-resistant isolates).
Implementation of PCV13 will increase the coverage of children in our area for invasive pneumococcal serotypes by approximately 30%. Postlicensure serotyping and antimicrobial susceptibility surveillance are essential for designing rational immunization strategies.