Background: Invasive Group B Streptococcus (Streptococcus agalactiae) remains a leading cause of infant morbidity and mortality. Intrapartum antibiotic prophylaxis (IAP) has been implemented in many countries with reduction in early onset disease, but an effective vaccine may further reduce disease burden. Candidate vaccines targeting capsular polysaccharides and surface proteins are now in clinical trials.

Methods: Using whole genome sequencing and phenotypic antimicrobial sensitivity testing, we characterised sterile site GBS isolates recovered from Western Australian infants between 2004-2020. Characteristics were compared between three time periods: 2004- 2008, 2009-2015, and 2016-2020.   Results: A total of 135 isolates were identified. Proportion of serotype III (22.7% in period 1 to 47.9% in period 3, p=0.04) and clonal complex 17 (13.6% to 39.6%, p=0.01) isolates increased over time. Overall coverage of vaccines currently being trialled was >95%. No isolates were penicillin resistant (MIC >0.25), but 21.5% of isolates had reduced penicillin susceptibility (MIC >0.12) and penicillin minimum inhibitory concentration (MIC) increased significantly over time (p=0.04). Clindamycin resistance increased over time to 45.8% in the latest period.

Conclusions: Based on comprehensive characterisation of invasive infant GBS in Western Australia, we found that coverage for leading CPS and surface protein vaccine candidates was high. The demonstrated changes in serotype and molecular type highlight the need for ongoing surveillance, particularly with regard to future GBS vaccination programs. The reduced susceptibility to IAP agents over time should inform changes to antibiotic guidelines.