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

OBJECTIVE: To describe the epidemiological characteristics, public health response, and evidence of international spread of a large measles outbreak that contributed to the loss of Canada's measles elimination status. DESIGN: Descriptive analysis of the cohort of measles outbreak cases reported to Ontario's public health surveillance system during the outbreak period: epidemiological week of 28 October 2024 to 6 October 2025. SETTING: Ontario, Canada. PARTICIPANTS: Individuals meeting the outbreak case definition for confirmed or probable measles infection during the outbreak period. EXPOSURE: Exposure to measles virus associated with the Ontario outbreak and related transmission chains. MAIN OUTCOME MEASURES: Demographic characteristics, vaccination status, admission to hospital, complications, pregnancy associated outcomes, mortality, and evidence of international spread based on measles virus genotyping and sequencing. Cases were identified using an outbreak-specific case definition incorporating epidemiological, clinical, laboratory, and viral sequencing criteria. Phylogenetic analysis of measles virus sequences was used to assess evidence of onwards spread to other countries. RESULTS: The outbreak consisted of 2377 cases (2064 confirmed and 313 probable) over 49 weeks and contributed to the loss of Canada's measles elimination status. Most cases (2146/2377, 90.3%) occurred among members of historically under-vaccinated communities, with limited spillover into the broader population. Among the 2377 cases, approximately three quarters of cases (n=1736, 73.0%) occurred in individuals aged 19 years or younger, 2121 (89.2%) were unvaccinated, and 97 (4.1%) had unknown vaccination status. The outbreak was associated with 166 (7%) admissions to hospital, 13 (<1%) admissions to intensive care unit, 51 (2.1%) infections during pregnancy, 10 (<1%) congenital measles infections, one (<1%) case of encephalitis, and one (<1%) infant death. Sequencing identified a novel N450 distinct sequence identifier that was subsequently detected in countries including the United States of America, Mexico, and the Netherlands. The outbreak strain was designated a World Health Organization named strain (MVs/Ontario.CAN/47.24). CONCLUSIONS: These findings highlight the risk of measles re-establishment in countries that have attained elimination and underscore the importance of culturally appropriate community engagement to build trust in immunisation and prevent future outbreaks. This outbreak shows the fragility of measles elimination in the presence of long standing immunity gaps within interconnected communities across the Americas and the population-level protection that limited broader community transmission.