Nasopharyngeal carriage of Streptococcus pneumoniae in healthy children aged less than five years

Purpose: In Turkey, pneumococcal conjugate vaccine (PCV) was introduced to the national immunization program as PCV7 in 2008, and was replaced with PCV13 in 2011. The aim of the study was to demonstrate the pneumococcal carriage rate and the serotype distribution in healthy children under 5 years in Turkey who were vaccinated with PCV13.

Methods: We conducted a cross-sectional study including the collection of questionnaire data and nasopharyngeal (NP) specimens among children aged <5 years from five centers from March 2019 to March 2020. Pneumococcal isolates were identified using optochin sensitivity and bile solubility. Serotyping was performed using a latex agglutination kit and Quellung reaction.

Results: NP swab samples were collected from 580 healthy children. The observed overall carriage rate was 17.8%. None of the hypothesised predictors of S. pneumoniae carriage, except maternal education level was statistically significant (p = 0.017). High maternal education level appeared to decrease the risk (lower vs. higher maternal education OR: 1.992 [95% CI; 1.089-3.643], p = 0.025). The overall NP S. pneumoniae carriage prevalence for the PCV13-vaccinated children was 17.8% (103/580). The most common serotypes detected were serotype 15B (n = 10, 9.7%), serotype 23F (n = 9, 8.7%), serotype 23A (n = 9, 8.7%), serotype 11A (n = 7, 6.7%), serotype 19F (n = 5, 4.8%) and serotype 15F (n = 5, 4.8%). Of the isolates, 28 (27.2%) were in PCV13 vaccine strains (VSs), and 75 (72.8%) strains were non-VS. The serotype coverage rate was 27.2% for PCV13.

Conclusion: The overall S. pneumoniae carriage rate was higher than in earlier studies from Turkey. Post-vaccine era studies from around the world have reported a decrease in VS serotypes and a 'serotype replacement' to non-VS serotypes, as we determined in our study.