Novel pneumococcal serotypes 6C and 6D: anomaly or harbinger

Clin Infect Dis. 2012 Nov 15;55(10):1379-86. doi: 10.1093/cid/cis691. Epub 2012 Aug 16.


Clinical use of the 7-valent pneumococcal protein conjugate (PCV7) vaccine, which includes serotypes 4, 6B, 9V, 14, 18C, 19F, and 23F, dramatically reduced invasive pneumococcal disease (IPD); however, the effectiveness was diminished due to serotype shift. Although shift due to known serotypes was anticipated, shift by misidentified serotypes was unexpected. We describe the experience with newly recognized serotypes 6C and 6D, which were mistyped as serotypes 6A and 6B, respectively. Although serotype 6D caused only occasional infections, IPD due to serotype 6C disease expanded in the PCV7 era. Subsequent studies showed that PCV7 provided cross-protection against serotype 6A but not serotype 6C. The 13-valent pneumococcal protein conjugate (PCV13) vaccine, which includes PCV7 serotypes plus serotypes 1, 3, 5, 6A, 7F, 19A, may provide protection against IPD due to serotypes 6C and 6D. Regardless, this narrative illustrates the potential impact of unrecognized serotypes on the efficacy of a serotype-specific vaccine.

Publication types

  • Research Support, N.I.H., Extramural
  • Review

MeSH terms

  • Heptavalent Pneumococcal Conjugate Vaccine
  • Humans
  • Pneumococcal Infections / immunology
  • Pneumococcal Infections / prevention & control
  • Pneumococcal Infections / virology*
  • Pneumococcal Vaccines / administration & dosage
  • Pneumococcal Vaccines / immunology
  • Serotyping
  • Streptococcus pneumoniae / classification*
  • Streptococcus pneumoniae / immunology


  • 13-valent pneumococcal vaccine
  • Heptavalent Pneumococcal Conjugate Vaccine
  • Pneumococcal Vaccines