Effect of the 7-valent pneumococcal conjugate vaccine on nasopharyngeal colonization by Streptococcus pneumoniae in the first 2 years of life

Background. Studies suggest that the 7- valent pneumococcal conjugate vaccine ( PCV7) reduces carriage of vaccine- type ( VT) Streptococcus pneumoniae ( SP). We studied the effect of PCV7 on carriage of VT- and non- VT ( NVT) SP, by studying the effect of PCV7 on nasopharyngeal ( NP) colonization by VT and NVT SP during early childhood. Methods. At 2 months of age, 278 infants were enrolled in this study. To determine carriage of SP, NP samples were obtained before each PCV7 dose, at 9 months of age, and 2 - 3 months after the booster dose of vaccine. Results. The carriage of SP increased slightly, from 12% ( 95% confidence interval [ CI], 8% - 16%) of subjects at 2 months of age to 18% ( 95% CI, 13% - 23%) at 4 months of age (P <.05). Carriage of SP remained in 24% - 30% of subjects during subsequent months. Between the 12- and 18- month visits, the carriage rate of VT SP decreased significantly, from 18% ( 95% CI, 13% - 23%) to 9% ( 95% CI, 5% - 13%) of subjects (P = .001). The trend of a decrease in carriage of penicillin- nonsusceptible SP, from 16% of subjects ( 95% CI, 11% - 21%) at the 12 - 15- month visit to 9% ( 95% CI, 5% - 13%) at the 15 - 18- month visit, was found after the booster dose of vaccine. Conclusion. The reduction of VT- SP colonization and replacement by NVT SP after the booster dose of vaccine suggests the possibility that widespread vaccination will result in replacement of pneumococci mainly by antibiotic- susceptible NVT SP.