This descriptive cohort study covered 2377 measles cases reported to Ontario's surveillance system from October 2024 to October 2025. Cases met an outbreak definition combining epidemiological, clinical, laboratory and viral sequencing criteria. Most cases (90.3%) were in historically under-vaccinated communities; 73.0% were aged 19 or under and 89.2% were unvaccinated. There were 166 hospital admissions (7%), 13 ICU admissions, 51 infections in pregnancy, 10 congenital infections and one infant death. Sequencing of the N450 genotyping region found a previously unseen variant, later detected in the USA, Mexico and the Netherlands. The analysis is descriptive only, and vaccination status was unknown for 4.1% of cases.
Why it is interesting: Measles became re-established in a country that had achieved elimination, with transmission concentrated in under-vaccinated communities. Viral sequencing traced the outbreak strain to several other countries, and WHO gave it a formal reference name, MVs/Ontario.CAN/47.24.
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