Wastewater Surveillance to Track Resurgent Measles Outbreak, Ontario, Canada, 2025

In 2025, the province of Ontario, Canada, experienced a major resurgent outbreak of measles with approximately 2,400 confirmed cases. A research team conducted wastewater surveillance concurrently with clinical surveillance to track measles incidence in southwestern Ontario. The measles virus signal in wastewater was positively associated with clinical cases but did not provide early warning of changes in measles incidence. Researchers found that measles virus RNA was distributed in the liquid phase but was most concentrated in solids. They developed an assay to differentiate vaccine and wild-type measles virus, which was used to detect vaccine genotype after a vaccination campaign targeting underserved groups. Canada lost its measles elimination status in November 2025 after 12 months of sustained transmission, marking the first loss of this status since 1998.