Objective:

To determine the serotype distribution, antibiotic susceptibility, and vaccine serotype coverage of Streptococcus pneumoniae isolates from adults in Türkiye between January 2019 and August 2025.

Approach:

• Study Design: A prospective, multicenter, noninterventional laboratory study involving 27 tertiary hospitals across 17 cities.
• Sample Collection: Researchers evaluated 462 isolates from adults aged 18 years or older, including invasive specimens and respiratory samples from patients meeting criteria for pneumococcal pneumonia. Only one isolate per patient was included.
• Microbiological Analysis: Identification was confirmed using optochin and bile lysis tests. Serotyping used the Quellung test.
• Antibiotic Susceptibility Testing: E-test results were interpreted using Clinical and Laboratory Standards Institute criteria, with separate meningitis and nonmeningitis breakpoints for parenteral penicillin and cefotaxime.

Key Findings:

• The most frequent serotypes were 3 (17.3%), 9N (9.1%), and 19F (5.4%).
• Serotype coverage was 34.6%, 35.9%, and 47.8% for the 13-, 15-, and 20-valent pneumococcal conjugate vaccines, respectively, and 59.5% for the 23-valent pneumococcal polysaccharide vaccine.
• Using nonmeningitis breakpoints, penicillin susceptibility was 96.3% and cefotaxime susceptibility was 97.8%.
• Erythromycin nonsusceptibility increased from 33.3% in 2019 to 57.5% in 2025.

Interpretation:

The 20-valent conjugate vaccine provides broader potential serotype coverage than the 13-valent vaccine. Coverage is higher among isolates from patients aged 65 years or older. These findings describe serotype coverage, not demonstrated vaccine efficacy.

Limitations:

• Only culture-positive cases were included, potentially biasing serotype representation.
• Clinical information was limited, and participants’ vaccination status was unavailable.
• Serotyping was incomplete for 78 isolates, potentially affecting coverage estimates.

Conclusion:

Serotype 3 predominates among the studied isolates. Continued surveillance and prudent antibiotic use remain essential, while broader vaccine serotype coverage may inform adult vaccination strategies.