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.