BMJ study details the Ontario measles outbreak that cost Canada its elimination status: 2,377 cases, 89% unvaccinated

A study published in The BMJ on 28 October 2026 gives the most detailed account yet of the Ontario measles outbreak that helped Canada lose the measles elimination status it had held since 1998. The Pan American Health Organization confirmed that loss in November 2025, after the same strain had circulated for more than 12 months.

What the researchers found

The team analysed every case reported to Ontario’s surveillance system between the week of 28 October 2024 and 6 October 2025 (49 weeks):

• 2,377 cases (2,064 confirmed, 313 probable); 90.3% were in historically under-vaccinated communities, with limited spillover to the wider population.
• 73% were aged 19 or younger; 89.2% were unvaccinated and 4.1% had unknown status.
• 166 hospital admissions (7%), 13 intensive-care admissions, 51 infections during pregnancy, 10 congenital measles infections, one case of encephalitis and one infant death.
• Sequencing identified a new strain, later designated by WHO as MVs/Ontario.CAN/47.24, that was subsequently detected in countries including the United States, Mexico and the Netherlands.

A linked BMJ editorial notes this outbreak accounted for nearly half of Canada’s more than 5,000 cases in 2025. On 19 October 2026 the Public Health Agency of Canada declared the wider multi-jurisdictional outbreak (more than 6,400 cases in 10 provinces and territories) over, but elimination status can only be restored after 12 consecutive months without transmission of the strain.

Limitations

This is a descriptive surveillance study: it relies on reported cases, so mild or unreported infections are missed, vaccination status was unknown for some, and it cannot test which interventions worked. The authors stress culturally appropriate community engagement to build trust in vaccination. Two doses of measles vaccine provide strong protection — check your records with your doctor or local public health unit.