Indirect Effect of Pneumococcal Conjugate Vaccines on Pneumococcal Colonization: Persistence and Dynamics of Vaccine Serotypes in Sicily (Italy) 11 Years Postintroduction, 2009–2020

Background In Italy, evidence on the long-term effects of pneumococcal conjugate vaccines on nasopharyngeal carriage is still limited. This study assessed pneumococcal carriage prevalence, serotype distribution, and temporal trends during the decade after 13-valent pneumococcal conjugate vaccine (PCV13) introduction and before the severe acute respiratory syndrome coronavirus 2 (COVID-19) pandemic.Methods Oropharyngeal samples were collected from 12 733 individuals of all ages presenting with influenza-like illness within the national respiratory pathogens surveillance network. Streptococcus pneumoniae detection and serotyping were performed using real-time PCR-based assays.Results Overall pneumococcal carriage was 27.1%. The highest prevalence occurred in children aged 2-4 years (51.6%), while colonization was about 10% among adults, including those >= 75 years. After vaccine introduction, PCV serotypes declined markedly, accompanied by increased nonvaccine serotypes. Following years of sustained pediatric vaccination, vaccine serotypes re-emerged, replacing previously expanding non-PCV types. Some vaccine serotypes associated with higher invasive disease risk persisted despite high vaccination coverage. Serotype distribution differed significantly by age, and viral coinfection-especially hRSV-appeared to increase pneumococcal colonization likelihood.Conclusions Pneumococcal carriage remained common across all ages despite long-standing pediatric vaccination, with continued circulation of both vaccine and nonvaccine serotypes. Viral coinfection may facilitate colonization, highlighting the need for ongoing surveillance and adaptive vaccination strategies.From 2009 to 2020, pneumococcal carriage remained substantial across all ages despite sustained pediatric vaccination. Vaccine serotypes initially declined but later re-emerged, replacing previously expanding non-PCV types. Persistent PCV serotypes underscore evolving pneumococcal dynamics and the need for continued surveillance.