Comparison of genotypes of Streptococcus pneumoniae serotypes 6A and 6B before and after the introduction of PCV7 vaccination in Korea

We investigated changes of serotypes in Streptococcus pneumoniae and the characteristics of serotypes 6A and 6B in Korean hospitals after the introduction of the 7-valent pneumococcal conjugate vaccine (PCV7). PCV7 serotypes decreased significantly from 66.8% in 1996-2001 to 41.3% in 2008-2009 (P < 0.001). However, serotype 6B did not decrease during these 2 periods (5.7% to 6.4%). Non-PCV7 serotypes that increased significantly were 3 (3.6% to 9.1%), 6A (3.1% to 8.5%), 13/28 (0 to 3.7%), 15 (2.2% to 6.7%), 16.36.37 (0 to 1.2%), 19A (1.2% to 9.7%), and 23A (0 to 1.5%). Multilocus sequence typing indicated that 9 STs were identified among 36 isolates of serotype 6A and 19 among 35 isolates of serotype 6B. No STs were found both in the 2 serotypes. The increase in serotype 6A was associated mainly with ST81, a Spain-23F clone genotype. The association of serotype 6A and ST81 is unique to Korea. Clones of serotype 6B isolates changed between the 2 periods. ST282, the main clone of serotype 6B in the PCV7 period, was found only in Korea. Our study demonstrates changing characteristics of pneumococcal serotypes 6A and 6B isolates in Korea and suggests selective evolution of certain clones in the era of PCV7 vaccination.