Microbiological and clinical characteristics of Streptococcus pneumoniae serotype 3 infection and risk factors for severe outcome: A multicenter observational study

Background/purpose: Serotype 3 has persisted to be an important cause of invasive pneumococcal disease in adults in the post-vaccine era. We aimed to investigate clinical and microbiological characteristics of Streptococcus pneumoniae serotype 3 infection in Taiwan and identify the risk factors associated with severe clinical outcome. Methods: A multicenter observational study was conducted to analyze serotype 3 isolates collected between 2012 and 2021. Demographics, comorbidities, and risk categories were statistically compared with clinical outcome. Antimicrobial susceptibility testing and multilocus sequence typing were performed. Results: A total of 146 isolates were collected, including 12 isolates regarded as colonizers. Among 134 infected cases, 54 (40.3%) were aged 65 and older. Mortality was significantly associated with diabetes mellitus, immunosuppression, immunodeficiency, high-risk status, and older age. Susceptibility rates were high to levofloxacin (98.9%), moxifloxacin (100%), vancomycin (100%), and ceftriaxone (97.3%). 25.3% (37/146) of the isolates showed intermediate susceptibility and 0.7% (1/146) showed resistance to penicillin. ST180 was the dominant sequence type. ST13 and ST9625 isolates were less susceptible to penicillin and ceftriaxone. Conclusions: Serotype 3 infection showed a high mortality rate, especially in patients with older ages and comorbidities. Although the incidence rates decreased during the COVID-19 pandemic, serotype 3 remained as an important cause of infection after the implementation of PCV13. Developing a more effective vaccine against serotype 3 and monitoring the antimicrobial-resistant sequence types are necessary.