Serotype Distribution and Antimicrobial Resistance of Streptococcus pneumoniae Causing Invasive Pneumococcal Disease in Korea Between 2017 and 2019 After Introduction of the 13-Valent Pneumococcal Conjugate Vaccine

Background: Streptococcus pneumoniae is a serious pathogen causing various infections in humans. We evaluated the serotype distribution and antimicrobial resistance of S. pneumoniae causing invasive pneumococcal disease (IPD) after introduction of pneumococcal conjugate vaccine (PCV)13 in Korea and investigated the epidemiological characteristics of multidrug-resistant (MDR) isolates. Methods: S. pneumoniae isolates causing IPD were collected from 16 hospitals in Korea between 2017 and 2019. Serotyping was performed using modified sequential multiplex PCR and the Quellung reaction. Antimicrobial susceptibility tests were performed using the broth microdilution method. Multilocus sequence typing was performed on MDR isolates for epidemiological investigations. Results: Among the 411 S. pneumoniae isolates analyzed, the most prevalent serotype was 3 (12.2%), followed by 10A (9.5%), 34 (7.3%), 19A (6.8%), 23A (6.3%), 22F (6.1%), 35B (5.8%), 11A (5.1%), and others (40.9%). The coverage rates of PCV7, PCV10, PCV13, and pneumococcal polysaccharide vaccine (PPSV)23 were 7.8%, 7.8%, 28.7%, and 59.4%, respectively. Resistance rates to penicillin, ceftriaxone, erythromycin, and levofloxacin were 13.1%, 9.2%, 80.3%, and 4.1%, respectively. MDR isolates accounted for 23.4% of all isolates. Serotypes 23A, 11A, 19A, and 15B accounted for the highest proportions of total isolates at 18.8%, 16.7%, 14.6%, and 8.3%, respectively. Sequence type (ST)166 (43.8%) and ST320 (12.5%) were common among MDR isolates. Conclusions: Non-PCV13 serotypes are increasing among invasive S. pneumoniae strains causing IPD. Differences in antimicrobial resistance were found according to the specific serotype. Continuous monitoring of serotypes and antimicrobial resistance is necessary for the appropriate management of S. pneumoniae infections.