Characterization of Streptococcus pneumoniae detected in clinical respiratory tract samples in South Sweden 2 to 4 years after introduction of PCV13

Background-objective: To determine the serotype distribution, antimicrobial resistance rates and relevance in invasive infections of Streptococcus pneumoniae present in the population of south Sweden, 2 to 4 years after the transition from the 10-valent pneumococcal conjugate vaccine (PCV10) to PCV13, we studied clinical samples from mucosal infections. Methods: All respiratory tract, middle ear, and conjunctival samples positive for growth of S. pneumoniae-positive were collected during an 18-month-period. In addition, clinical, demographical and antimicrobial susceptibility data were compiled. Corresponding isolates were serotyped by PCR and/or antisera (n=1,858). Whole genome sequencing was performed on a subset of the most antibiotic resistant isolates to determine molecular epidemiology (n=26). Results: In total, 17% (n=313) of the serotyped isolates were covered by PCV13, predominantly represented by serotypes 3 (9%) and 19A (5%). Serotypes included in PCV13 were more common in samples from hospital settings (OR 1.43; p=0.006) and less prevalent in samples from children of PCV13-eligible age (OR 0.46; p<0.001). The most common nonvaccine serotypes were 11A (10%), 23B (10%), 15A (6%) and 35F (5%). Isolates exhibiting 15A and 23B were often penicillin nonsusceptible (21%) and multidrug-resistant (38%), respectively. Frequently detected nonvaccine serotypes seldomly caused invasive infections according to regional surveillance data. Most pneumococci nonsusceptible to ≥5 antibiotics belonged to known global multiresistant lineages (19/26 sequenced isolates).Conclusions: Serotype 19A was two-fold less common compared to a previous observation period (years 2011-2013), suggesting emerging herd protection. In contrast, no overall decline of serotype 3 was observed, but a possible direct vaccine effect was noted in the PCV13-targeted age-group. Despite many increasing nonvaccine serotypes commonly associated with mucosal infections have low invasive potential, antibiotic resistant strains may pose a future clinical problem.