Impact of a large measles outbreak associated with transmission in the Americas and Canada's loss of measles elimination status: descriptive cohort study.

Ontario’s 49-week measles outbreak produced 2377 cases and contributed to Canada losing measles elimination status, with 90.3% occurring in historically under-vaccinated communities.

• Why it matters: Measles elimination can be fragile when immunity gaps persist and viruses spread internationally, creating a need to characterize outbreak dynamics and cross-border evidence.
• What they did: A descriptive cohort analysis used Ontario’s outbreak-specific confirmed/probable case definition for 28 October 2024 to 6 October 2025, including vaccination, hospital outcomes, pregnancy outcomes, and measles virus genotyping/sequencing with phylogenetic analysis.
• The result: Among 2377 cases, 73.0% were aged 19 years or younger and 89.2% were unvaccinated; 166 (7%) required hospital admission, and a novel N450 sequence was detected in the USA, Mexico, and the Netherlands.