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The outbreak that contributed to Canada losing its measles elimination status in November 2025 was concentrated in historically undervaccinated communities with close social networks across Canada, the U.S., Mexico and beyond, creating opportunities for cross-border transmission, concludes a study published by The BMJ.

The researchers say the findings highlight the ongoing risk of measles resurgence in countries that have previously successfully controlled the disease, and underscore the importance of culturally tailored community engagement to build trust and confidence in vaccination and prevent future outbreaks.

Canada achieved measles-free status in 1998 after introducing a two-dose vaccine schedule and a national catch-up program for school-aged children, but lost it in November 2025 after a measles outbreak that involved several provinces.

In Ontario, the outbreak disproportionately affected historically undervaccinated communities—groups with historically low vaccine coverage and close social networks across Canada, the U.S. and Mexico.

To better understand the course of this outbreak and its impact, researchers analyzed measles cases reported to Ontario's public health surveillance system during the outbreak period (Oct. 28, 2024, to Oct. 6, 2025).

The outbreak consisted of 2,377 cases (2,064 confirmed and 313 probable) over 49 weeks. Most cases (90%) occurred among members of historically undervaccinated communities, with limited spillover into the broader population.

Among the 2,377 cases, around three-quarters (73%) occurred in individuals aged 19 years or younger, 2,121 (89%) were unvaccinated and 97 (4%) had unknown vaccination status.

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