Indirect effect of pneumococcal conjugate vaccines on pneumococcal colonization: persistence and dynamics of vaccine serotypes in Sicily (Italy) eleven years post-introduction, 2009 to 2020.

Abstract

In Italy, evidence on the long-term impact of pneumococcal conjugate vaccines on nasopharyngeal carriage remains limited. This study investigates pneumococcal carriage prevalence, serotype distribution, and temporal trends in the decade following the introduction of PCV13 and preceding the onset of the COVID-19 pandemic (2009-2020). Oropharyngeal samples were collected from 12,733 individuals of all ages presenting with influenza-like illness within the national surveillance network for respiratory pathogens. Streptococcus pneumoniae detection and serotyping were performed using real-time PCR-based assays. Overall pneumococcal carriage was 27.1%. The maximum value was observed in children aged 2-4 years (51.6%), whereas colonization was about 10% in adults, including those aged ≥75 years. Following vaccine implementation, a marked decline in PCV-covered serotypes was observed, accompanied by the rise of non-vaccine serotypes. After several years of sustained pediatric immunization, vaccine serotypes re-emerged, replacing previously expanding non-PCV types. Some persistent vaccine serotypes, including those associated with a higher risk of invasive disease, continued to circulate despite high and longstanding vaccination coverage. Serotype distribution varied significantly by age, and concurrent viral infections, particularly with hRSV, appeared to increase pneumococcal colonization risk. Despite sustained high pediatric vaccination, pneumococcal carriage remained substantial across all ages, with persistent circulation of vaccine and non-vaccine serotypes. Viral co-infection, particularly with hRSV, appeared to facilitate colonization. These findings underscore the need for ongoing carriage surveillance and evolving vaccine strategies to address changing pneumococcal ecology.