Measles is highly contagious but is also highly preventable through vaccination. The respiratory disease primarily impacts young people and can cause severe outcomes, including the need for hospitalization. Although vaccination is accessible, safe, and effective, reduced vaccination in isolated communities continues to drive outbreaks.
Since October 2024, Canada has been impacted by an outbreak of measles, primarily in southwestern Ontario. After nearly two years, the Public Health Agency of Canada announced on October 19, 2026, that the measles outbreak has officially ended.
Understanding the cause of the outbreak on a population level can help with public health response to prevent further outbreaks in Canada and around the globe. A team of Canadian researchers analyzed public health data for measles cases during the first year of the outbreak, from October 28, 2024 to October 6, 2025. The team, led by Sarah Wilson, MD, Deputy Chief, Medical and Systems Support, Communicable Disease Control at Public Health Ontario, published their study in BMJ this week.
Over the course of the outbreak, Canada lost its measles elimination status that it had maintained since 1998. “Canada’s loss of measles elimination status is a troubling sign of a global trend in increasing susceptibility to infectious diseases,” wrote Jennifer Nuzzo, DrPH, director and professor at Brown University School of Public Health in a separate editorial also published in BMJ.
During the outbreak, Wilson and colleagues identified 2,377 cases of measles—2,064 confirmed and 313 probable. The team analyzed a variety of data associate with these cases, including demographic characteristics, vaccination status, admission to the hospital and complications, outcomes associated with pregnancy, and mortality. Using polygenic analysis of virus sequences, they were also able to assess evidence of international spread.
The majority of measles cases, approximately 90% (2146), “occurred among members of historically under‑vaccinated communities, with limited spillover into the broader population.” Further, a novel strain of the virus, N450, was found to have spread to multiple countries outside of Canada, including the U.S., Mexico, and the Netherlands.
Many cases—73%, 1736 individuals—also occurred in children 19 years old or younger. Vaccination assessment showed that 89% of the cases, 2121 individuals, were unvaccinated and an additional four percent, 97 individuals, had unknown vaccination status against measles.
Hospitalizations were identified in seven percent of cases, with less than one percent of patients admitted to the intensive care unit. 51 individuals, accounting for two percent of the infections, were pregnant at the time of infection, resulting in 10 congenital measles infections.
In addition to quantifying the outcomes of the measles outbreak, the team collected data on the public health response. Wilson and colleagues noted the response comprised a multifaceted approach including recommendations for self-isolation of symptomatic individuals and reduced contact with others during the incubation period.
Communications to the public were tailored to the communities most effected, with translations into multiple languages as needed. These messages were disseminated through multiple media outlets including radio, newspapers, and WhatsApp groups and “emphasized personal autonomy, protecting the vulnerable by preventing onward transmission (e.g., avoiding travel while ill), increasing awareness of measles symptoms and complications to promote timely presentation to health care, and providing reasons to choose immunization.”
The researchers also report that healthcare workers described a number of challenges during the outbreak, “including language barriers and misunderstandings during healthcare encounters, concerns about stigma and stereotyping,…and mistrust of government institutions.” While the outbreak in Canada was high in the Mennonite community, these public health challenges are not unique to this community and similar concerns are found in other tight-knit communities world-wide.
“Strengthening of immunization systems, improving vaccine coverage, and sustaining culturally appropriate engagement to continue to build trust in immunization will be essential to prevent future vaccine preventable disease outbreaks and to support re-establishing measles elimination,” concluded the authors.
“The return of measles in countries previously designated as eliminated is a warning about global infectious disease risk,” Nuzzo concurred. “Countries need to recommit to measles elimination; not only to restore a designation but also to stop a dangerous, preventable disease and to strengthen the public health systems that protect their own populations and those of their neighbors from other serious infectious threats.”
