The importance of active laboratory surveillance of invasive pneumococcal diseases in assessing the effects of vaccination against pneumococcal diseases in Serbia

Data on the circulating serotypes of Streptococcus pneumoniae and their antibiotic resistance are essential in the study of the seroepidemiology of S. pneumoniae. In Serbia, although legally mandated, active surveillance of invasive pneumococcal diseases has not been effectively implemented in practice. The aim of this study is to present the invasive strains of S. pneumoniae identified five years before and five years after the introduction of mandatory pneumococcal vaccination in Serbia, as well as their antibiotic susceptibility, based on data from the General Hospital "Dr Laza K. Lazarević" in Šabac, and to emphasize the importance of monitoring the serotype distribution and the prevalence of antibiotic resistance. The study was conducted at the General Hospital "Dr Laza K. Lazarević" in Šabac and included all vaccinated and unvaccinated children up to 18 years of age who were diagnosed with and confirmed to have invasive pneumococcal disease during the period from January 1, 2013, to December 31, 2022. Data were collected from electronic medical records and documentation from the microbiology laboratory. Diagnosis was established based on blood cultures positive for S. pneumoniae. The bacterium was identified using standard bacteriological procedures. The serotyping of S. pneumoniae isolates was performed at the National Reference Laboratory (NRL) for Streptococcus at the Institute of Microbiology and Immunology, Faculty of Medicine, University of Belgrade. Antibiotic susceptibility was tested using the disk diffusion method and E-test. During this ten-year period, five cases of invasive pneumococcal disease were registered. Three different serotypes were identified: 4, 18C, and 19A, while one strain was non-typable. One isolate was susceptible to all tested antibiotics, while the others showed resistance to at least one antibiotic from the beta-lactam group. Two isolates exhibited combined resistance to beta-lactam antibiotics and clindamycin, while one demonstrated combined resistance to benzylpenicillin and trimethoprim-sulfamethoxazole. All tested isolates were susceptible to macrolide antibiotics, ciprofloxacin, levofloxacin, vancomycin, teicoplanin, and linezolid. To effectively plan the national immunization program and optimize treatment protocols, active surveillance of invasive pneumococcal diseases and the circulating S. pneumoniae serotypes in Serbia is necessary, as well as more rational use of antibiotics in order to reduce resistance.