Objectives
To characterize the recent trends in serogroup W isolates from invasive meningococcal disease (IMD) cases (MenW) in Ontario, Canada since 2015.

Methods
IMD case isolates in Ontario between January 1, 2015 and June 30, 2024 were examined by phenotypic and genetic methods for possession of vaccine antigen genes and clonal characteristics. MenW ST-11 clonal complex (CC) strains were compared against global MenW isolates by core-genome multi-locus sequence typing (cgMLST).

Results
The percentage of culture-confirmed IMD caused by MenW in Ontario increased from 10 % in 2015–40.9 % in the first half of 2024, consisting entirely of strains belonging to the ST-11 CC. cgMLST comparison of the Ontario invasive MenW isolates versus international MenW ST-11CC strains showed that the Ontario isolates were related to those found globally, with a recent cluster of eight cases from one city due to a strain highly related to international Umrah outbreak strains. Most MenW IMD cases (60 %) occurred in individuals older than 40 years of age and the majority (83.3 %) predicted to express antigens covered by the 4CMenB vaccine.

Conclusions
Multiple different introductions of international MenW strains likely accounted for the recent shift towards invasive MenW disease in Ontario.

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