Pneumococcal serotype 3 otitis media, limited effect of polysaccharide conjugate immunisation and strain characteristics

Background: In contrast to the other vaccine serotypes, no protection could be demonstrated in the POET study against serotype 3 acute otitis media (AOM) following primary and booster vaccination with a multi-valent pneumococcal conjugate vaccine. Methods: AOM efficacy and immunogenicity data were reviewed. Pheno- and genotypic characteristics of different serotype 3 strains including efficacy Study AOM isolates were evaluated. Results: Evaluation of vaccine efficacy before and after booster vaccination indicated that lack of efficacy against serotype 3 pneumococci Might have been due to declined protection following the booster dose. However, although atypical immunogenicity was observed for serotype 3 in the second year of life, the capacity to respond to serotype 3 plain polysaccharide was not impaired. All but one of the serotype 3 strains examined had abundant polysaccharide capsules. Comparison of serotype 3 Capsular polysaccharide biosynthesis gene sequences found no relevant differences between any of the serotype 3 strains, but mRNA transcript levels were lower for the less densely encapsulated strain. Conclusion: Lack of clinical efficacy against serotype 3 AOM following pneumococcal conjugate vaccination may be due to an impaired induction of immune memory. A possible alternative explanation may lie with the atypically abundant expression of capsular polysaccharide which could make serotype 3 strains less susceptible to anti-polysaccharide antibody defence mechanisms in the middle ear. The occurrence of a capsular forms during biofilm growth may also play a role. Clinical impact against otitis media, of vaccines containing pneumococcal serotype 3 components, remains unclear until further investigations have demonstrated the value. (C) 2009 Elsevier Ltd. All rights reserved.