Despite the concern of replacement disease, notably by serotype 19A after 7-valent conjugate vaccine (PCV7) use, serotype 19A was increasingly recognized in Korean children before the introduction of PCV7. To understand the dynamics of serogroup 19 prevalence from 1991-2006, we serotyped 538 pediatric pneumococcal isolates. Serogroup 19 isolates (n = 126) were characterized by antimicrobial drug susceptibility, presence of mefA/ermB, and multilocus sequence typing. Overall, the proportion of serotype 19A isolates increased but serotype 19F decreased. Among children <5 years of age, the proportion of serotype 19A isolates in invasive pneumococcal disease increased from 0% in 1991-1994 to 8%-10% in 1995-2000, reached 26% in 2001-2003, and remained at 20% in 2004-2006 when vaccine coverage did not exceed 25% (p = 0.005 for trend). This study demonstrates that the expansion of multidrug-resistant ST320 was responsible for the increase in serotype 19A before PCV7 use.
Summary Streptococcus pneumoniae is a major cause of invasive infections in young infants and children. Since the introduction of 7-valent conjugate vaccine (PCV7) in the United States, a decrease in the incidence of invasive pneumococcal disease (IPD) caused by vaccine serotypes has been observed in pediatric and nonpediatric age groups. However, the incidence of IPD caused by nonvaccine serotypes (including serotype 19A) increased. The demonstration of an increase in 19A before the use of PCV7 in South Korea showing in this study suggests that PCV7 vaccination may not be entirely responsible for the observed increase of serotype 19A in South Korea. Therefore, surveillance is essential to monitor antimicrobial drug resistance, serotype expansion, and serotype replacement as early indications of an increase in pneumococcal disease by non-PCV7 or PCV7-related serotypes.