Most studies evaluating the impact of PPS immunization in the absence of additional PCV in infants or children have not shown any impact on pneumococcal disease or carriage [17], [46] and [47]. In contrast, a study in Papua New Guinea, where children aged six months to five years of age were given either the 14 or 23vPPS in one or two doses according to age, there was a (non-significant) 19% reduction in mortality from any cause, and a 50% reduction in pneumonia mortality (95%CI 1–75%) [48]. Natural exposure in a population with

a high incidence of pneumococcal infections, resulting in regular antigenic Trametinib manufacturer stimulation may explain this finding [20]. However, a Finnish study of the 14-valent PPS in infants aged three months to six years showed significant efficacy against vaccine type recurrent otitis media was 52% for children less than two years of age if serogroup 6 was excluded [13]. A study documenting immunological memory five years after meningococcal A/C conjugate vaccination

in infancy showed that challenge with the meningococcal polysaccharide or conjugate at two years of age induced immunological memory [21]. Ruxolitinib However, subsequent challenge with polysaccharide at five years of age failed to induce a similar memory response in the polysaccharide group. The authors concluded that the initial polysaccharide immunization at two years of age interfered with the immune response to subsequent polysaccharide vaccination, a finding similar to our current results with 23vPPS [21]. No adverse clinical effects have ever been documented from repeated exposure to the meningococcal polysaccharide vaccine and in this study we demonstrated no increase in clinical adverse effects to the 23vPPS, although the numbers were small and the study was not designed to study

this. There was no increase in nasopharyngeal carriage of non-PCV serotypes five months after receipt of the 12 month 23vPPS (FMR, isothipendyl JRC, EKM). We intend to follow the children from this study to assess nasopharyngeal carriage as an increase in carriage of non-PCV types in the 12 month 23vPPS group would indicate that this immunological finding may have a biological effect. This would provide the first indication that these children may have increased susceptibility to pneumococcal disease. Further results documenting the avidity and opsonophagocytic activity post 23vPPS and mPPS, and the impact on nasopharyngeal carriage will follow. In addition, immunological assays to assess B cell subsets will enable a more comprehensive assessment of the impact of 23vPPS on immunological functioning. However, our findings suggest that additional immunization with the 23vPPS following a primary series of PCV does not provide added benefit for antibody production and instead results in impaired immune responses following a subsequent PPS antigen challenge. Whether this observation is associated with adverse clinical effects remains to be determined.