Impact of sequential pediatric pneumococcal conjugate vaccination (PCV10/PCV13/PCV10) on serotype distribution and antimicrobial resistance in invasive and non-invasive Streptococcus pneumoniae isolates in Serbian children

Background: In Serbia, mandatory PCV immunization was introduced in 2018, following a sequential PCV10/PCV13/PCV10 schedule. This study aimed to assess changes in pneumococcal serotype distribution, vaccine serotype coverage, antimicrobial resistance (AMR), and clonal dissemination of pediatric invasive and non-invasive isolates before and after PCV introduction. Methods: A total of 626 clinical pneumococcal isolates from children were analyzed, including 251 invasive (IPD) and 375 non-invasive (NIPD) isolates. Serotyping was performed using PCR and Quellung reactions, antimicrobial susceptibility testing followed EUCAST guidelines, and invasive isolates were further characterized by multilocus sequence typing (MLST). Serotype invasiveness ratios were evaluated in pre- (2010-2017) and post-PCV (2018-2024) periods. Results: Following PCV implementation, serotype coverage declined across all vaccines, most markedly for PCV10 (IPD: 67.7-35.6%; NIPD: 62.3-17.4%), with secondary reductions in PCV13, PCV15, and PCV20 coverage largely driven by the loss of PCV10 serotypes. The proportion of non-PCV20 serotypes rose from 7.2 to 22.9% in IPD and from 12.3 to 41.6% in NIPD, with NVTs being significantly more frequent in NIPD post-PCV period. Antimicrobial non-susceptibility decreased after PCV introduction, including significant reductions in penicillin and macrolide resistance in both IPD and NIPD isolates. Resistance levels remained higher among non-invasive isolates compared with invasive isolates. Multidrug resistance declined from 39.8 to 25.4% in IPD and from 61.0 to 29.5% in NIPD. MLST analysis showed broad clonal diversity, with ST473 and ST8144 predominating pre-PCV and ST1377 and ST320 post-PCV, together with the emergence of non-vaccine lineages and serotype switch in ST320 from 19F to 19A. Conclusion: Mandatory PCV immunization in Serbia has led to substantial shifts in pneumococcal serotype distribution, resembling patterns observed in countries relying predominantly on PCV10. Also, a reduction in antimicrobial resistance and an increase in genetic diversity were observed, highlighting ongoing serotype replacement and the need for continued integrated surveillance.