Changes in the Streptococcus pneumoniae population responsible for invasive disease of young children after the implementation of conjugated vaccines in the National Immunization Program in Poland

Background: In Poland, 10-valent pneumococcal conjugate vaccine (PCV10) was added to the National Immunization Program (NIP) in 2017. We aimed to investigate the population structure and genomic composition of pneumococcal strains responsible for invasive infections in young children before and after the introduction of PCV10 to the NIP. Methods: The study encompassed all laboratory-confirmed cases of invasive pneumococcal disease between 2014 and 2020 in Polish children under the age of two. This period was split into two phases: the pre-PCV period (2014–2016) and the post-PCV period (2017–2020). Standard methods were utilized for identifying, serotyping, and testing the antimicrobial susceptibility of the isolates. All S. pneumoniae isolates underwent whole genome sequencing and analysis. Results: Following the introduction of PCV10, the prevalence of PCV10 vaccine-type (PCV10-VT) strains decreased significantly from 56.9 % to 29.5 %. Concurrently, the additional three serotypes present in higher valency PCVs (3, 6A and 19A), increased from 17.6 % to 25.7 %, and non-vaccine serotypes from 12.8 % to 22.9 %. A significant decline was observed in resistance to penicillin (49.0 %–34.3 %), and erythromycin (55.9 %–41.9 %), in the percentage of multidrug-resistant (46.1 %–30.5 %), and pilliated isolates (50.0 %–28.6 %). In the pre-PCV period, Global Pneumococcal Sequence Cluster (GPSC), GPSC1 was the most prevalent (17.6 %), and in the post-PCV period GPSC12 emerged as the most common (10.5 %). Changes in the GPSC composition within particular serotypes (e.g. 14 and 19A) were also observed. Conclusions: In the first three years after the introduction of the PCV10 into the Polish NIP significant reduction in the prevalence of PCV10-VTs was observed. Molecular characterization of the studied population allowed us to link the observed changes in the GPSC prevalence, with a decrease of infections caused by multi-drug resistant pneumococci and those with higher virulence associated with the presence of pili. The consistent evolution of S. pneumoniae influenced by population-based vaccinations, requires continuous surveillance.