Outbreaks of pneumococcal infections have been reported in closed settings and are often caused by clusters of genomically highly related isolates (genomic clusters), suggesting close transmission connections.
We used whole-genome sequencing (WGS) to characterize all IPD isolates identified through the Active Bacterial Core surveillance (ABCs) from 2015 to 2017. We identified genomic clusters by performing hierarchical cluster analysis of pair-wise genomic distances and applying a cut-off value of 3-base difference per 1.8Mb shared genome.
WGS characterized 8029 isolates representing 87% of all IPD cases. The cluster analysis identified 379 genomic clusters accounting for 847 (11%) of the isolates. Higher proportions of clustered isolates (isolates belonging to any clusters) were found in serotypes 12F (36%), 4 (26%), and 7F (25%), while lower proportions were found in serotypes 35F, 7C, 15A, 23B (each <1.5%), and penicillin non-susceptible isolates (4%). Clustered isolates were significantly associated with patients who were homeless (OR=3.3), who were injecting drugs (OR=2.1), and who were 25 to 50 years of age (OR=1.5; all p-values < 0.001).
IPD genomic clusters were associated with specific patient and strain features in the ABCs population. Understanding the dynamics and demographics of vaccine-serotype clusters could help identify new target groups to inform vaccine policy.