Increased proportions of invasive pneumococcal disease cases amongs adults experiencing homelessness sets stage for new serotype 4 capsular-switch recombinant.

Background: The Centers for Disease Control and Prevention's Active Bacterial Core surveillance (ABCs) identified increased serotype 4 invasive pneumococcal disease (IPD), particularly among adults experiencing homelessness (AEH).

Methods: We quantified IPD cases during 2016-2022. Employing genomic-based characterization of IPD isolates, we identified serotype-switch variants. Recombinational analyses were used to identify the genetic donor and recipient strains that generated a serotype 4 progeny strain. We performed phylogenetic analyses of the serotype 4 progeny and serotype 12F genetic recipient to determine genetic distances.

Results: We identified 30 inter-related (0-21 nucleotide differences) IPD isolates recovered during 2022-2023, corresponding to a serotype 4 capsular-switch variant. This strain arose through a multi-fragment recombination event between serotype 4/ST10172 and serotype 12F/ST220 parental strains. Twenty-five of the 30 cases occurred within Oregon. Of 29 cases with known residence status, 16 occurred in AEH. Variant emergence coincided with a 2.6-fold increase (57 to 148) of cases caused by the serotype 4/ST10172 donor lineage in 2022 compared to 2019 and its first appearance in Oregon. Most serotypes showed sequential increases of AEH IPD/all IPD ratios during 2016-2022 (for all serotypes combined, 247/2198, 11.2% during 2022 compared to 405/5317, 7.6% for 2018-2019, p<0.001). Serotypes 4 and 12F each caused more IPD than any other serotypes in AEH during 2020-2022 (207 combined reported cases primarily in 4 western states accounting for 38% of IPD in AEH).

Conclusion: Expansion and increased transmission of serotypes 4 and 12F among adults potentially led to recent genesis of an impactful hybrid "serotype-switch" variant.