High Prevalence of Non-vaccine Serotypes among Pediatric Non-invasive Pneumococcal Isolates in the PCV10 Vaccine-era in Bulgaria

Streptococcus pneumoniae is a vaccine-preventable agent, due to the available pneumococcal conjugate vaccines (PCVs), but still is a leading bacterial pathogen of non-invasive pneumococcal diseases (NIPD). We conducted a study on the serotype distribution and antibiotic nonsusceptibility in 202 S. pneumoniae isolates recovered from children with NIPD during the PCV10-era in Bulgaria (2015–2020). Serogrouping/serotyping were performed using latex agglutination and capsular swelling reaction. Serogroup 6 strains were subjected to serotype-specific PCR's. The antibiotic susceptibilities were assessed by broth microdilution. The PCV10-vaccinated children were 190 (94.1%). The antimicrobial non-susceptibility showed highest levels in oral penicillin (53.5%), erythromycin (52.0%), trimethoprim-sulfamethoxazole (48.5%), clindamycin (47.5%), tetracycline (45.0%), ceftriaxone (14.8%) and chloramphenicol (13.4%). More than half of the strains (53.5%) were MDR. We disclosed 80.2% non-vaccinal serotypes (NVTs) and 17.8% PCV10-serotypes. Except serotype 19F (19.4%), all emergent pediatric NIPD serotypes in our geographic area during the studied PCV10-period were NVTs: 19A (15.8%), 3 (14.8%), 6C (9.9%), 15A (6.0%) and 23A (4.0%). MDR-serotypes were 19A (22.2%), 19F (17.6%), 6C (15.7%), 15A (8.3%), and 23A (8.3%). Surveillance studies must be performed systematically to monitor the vaccine-induced changes and trends in antimicrobial resistance.