Carriage of Streptoccoccus pneumoniae 7 years after implementation of vaccination program in a population with very high and long-lasting coverage, Italy.

To evaluate how the 7-valent pneumococcal vaccine (PCV7) programme and the very high vaccination coverage reached for over 4 years affected the prevalence of Streptoccoccus pneumoniae serotypes in the paediatric population and to evaluate demographic, behavioural and risk factors for carriage in the post-vaccination era, a cross-sectional study on nasopharyngeal carriage was performed. Six hundred sixty-nine children under the age of 5, representative of the open population, were enrolled by cluster sampling. High sensitive techniques for detection of multi-serotype carriage, i.e. broth enrichment and real-time PCR and sequential PCRs for detection and typing, respectively, were used. Of the 669 enrolled children, 97.8\% were compliant with the recommended PCV7 vaccination schedule. Post-stratification adjustment for age was applied considering the Ligurian population as standard population. Age-weighted carriage rate was 50.1\% and 78\% of carriers were colonized by more than one serotype. The prevalence of carriage increased with age from 22\% in the first year of life, to 48.6\% in the second year of life and to 60\% in the 25-59 month age group. Age-weighted prevalence of any of the PCV7, PCV10 or PCV13 serotypes was 10.3\%, 20.3\% and 27.5\%, respectively. PCV7 serotypes were mainly represented by serotype 4 that was carried since the 3rd year of life and was responsible for invasive pneumococcal disease (IPD) and non-IPD in adults, but not in children confirming the high vaccine effectiveness. Among the serotypes included in recently available vaccines, serotypes 5 and 19A showed a higher prevalence, being carried by 15.2\% and 8.8\% of the population, respectively. A multivariate analysis showed that age, the presence of child siblings at home and day care attendance covariates were strongly associated with S. pneumoniae carriage. In conclusion, over 7 years of vaccination with PCV7 and very high coverage in the last 4 years has led to low carriage prevalence in the first year of life rapidly increasing in the following years and high prevalence of non-PCV7 serotypes carriage.