Prevalence of candidate vaccine targets and genomic features of pediatric invasive Streptococcus agalactiae in Japan.

BACKGROUND Streptococcus agalactiae (Group B Streptococcus, GBS) is a leading cause of invasive neonatal and infant infections, including sepsis and meningitis. Despite intrapartum antibiotic prophylaxis, GBS remains a public health concern. This study aimed to estimate the vaccine coverage and characterize the genomic epidemiology of pediatric invasive GBS in Japan. METHODS We conducted a nationwide, multicenter, retrospective genomic surveillance study involving 237 GBS isolates from sterile specimens of children aged ≤15 years across 35 hospitals in Japan between 2004 and 2023. Serotyping, antimicrobial susceptibility testing, whole-genome sequencing, and phylogenetic and single nucleotide polymorphism (SNP)-based analyses were performed. RESULTS Serotype III was the most common (48.1%), followed by Ia (27.4%) and Ib (12.2%). The estimated vaccine coverage was 98.3% for the hexavalent polysaccharide vaccine and 94.9% for the GBS-NN/NN2 protein vaccine. All isolates were susceptible to penicillin. Erythromycin and clindamycin resistance were observed in 61.2% and 43.5%, respectively. Among the 75 CC17 isolates, 59 (78.7%) contained only PI-2B and harbored both ermB and tetO, indicating the predominance of a multidrug-resistant clone. SNP-based analysis revealed evidence of nosocomial transmission and persistent regional circulation, particularly within the ST17 and ST23 lineages. CONCLUSION This nationwide genomic surveillance study suggests that current maternal GBS vaccine candidates would provide broad coverage for pediatric invasive infections in Japan. The identification of persistent and regionally disseminated lineages highlights the importance of investigating potential, yet poorly understood, transmission routes, including environmental reservoirs, to inform future prevention strategies.