Public health nurse Lauri Bidinot demonstrates how to give a measles shot at Southwestern Public Health in St. Thomas, Ont., in March. Experts say vaccination efforts have contributed to the downward trajectory of cases in Ontario.Geoff Robins/The Canadian Press
Infectious-disease experts are cautiously optimistic that Ontario’s measles outbreak will continue to taper off after a gradual decline in new cases over the past two months.
Cases peaked in early May, show provincial data, with weekly case counts hovering around 200. They have since been declining with some variation.
The province reported 21 new cases during the past week, according to Public Health Ontario’s weekly measles update on Thursday. In the previous week, the province reported 11 new cases.
Experts say the overall downward trajectory can be traced to several factors, such as widespread vaccination efforts, case management and the proportion of susceptible hosts becoming immune through infection or being immunized against measles.
Isaac Bogoch, an infectious-disease specialist at Toronto General Hospital, said it is difficult to predict how long the outbreak will last, partly because the size of the susceptible population is unclear. But he said the data clearly show a positive trend.
“You see in general, week after week, a significant drop in cases from its peak. It doesn’t mean it’s over, and it still can grumble along for many weeks or even months to come, but it’s an arrow pointing in the right direction,” he said.
Canada’s measles outbreak began last October in New Brunswick before spreading to Ontario, which has reported the highest number of cases in the country at more than 2,200. The first, and only, fatality linked to the outbreak is a premature baby born in Southwestern Ontario who was infected in utero.
But, while cases are easing in Ontario, another province is still seeing measles infections climb. Alberta, which has a population about one-third the size of Ontario, currently has the highest number of cases per capita in all of North America at a rate of 25.5 per 100,000 people.
Spiking cases in Alberta have prompted some hospitals to triage patients in their vehicles. Janet Laurie, a spokesperson for health care provider Covenant Health, said Grey Nuns Community Hospital in Edmonton requested patients with measles symptoms or exposure to be triaged in their car, “provided it is safe to do so.”
She said other sites in Alberta are following a similar protocol to reduce the potential exposure to patients and visitors in emergency departments.
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Canada declared measles eliminated in 1998 owing to widespread measles, mumps and rubella, or MMR, vaccine coverage. But the country is at risk of losing its status if the outbreak is not brought under control by the fall.
Cases in Ontario and Alberta have largely been among children who have not been vaccinated against measles. The same is true for the United States. On Thursday, new data from the U.S. Centers for Disease Control and Prevention show that the 2025 outbreak is the worst on record since the U.S. declared measles eliminated in 2000.
In Ontario, the Southwestern Public Health Unit, which covers a mostly rural swath of the province south and east of London, has been hardest-hit with measles cases.
Ninh Tran, the medical officer of health at Southwestern PHU, said he is “cautiously encouraged” by the recent decline in reported measles cases but said it is too soon to confirm a persistent pattern.
“The consistency of the decrease does suggest a potential shift in the trajectory of the outbreak and it may indicate that our public-health interventions – including vaccination efforts, community outreach and case management – are having a meaningful impact,” Dr. Tran said.
“However, we remain vigilant and continue to monitor the situation.”
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Kieran Moore, Ontario’s Chief Medical Officer of Health, declined an interview request. Dr. Moore has said that the provincial outbreak traces back to an exposure at a Mennonite gathering in New Brunswick last fall.
Zahid Butt, an infectious-disease specialist and professor at the University of Waterloo, said that in addition to efforts aimed at increasing vaccine coverage among school-aged children, the decline in cases could be linked to schools being out for the summer.
While he is “cautiously optimistic” that the worst of Ontario’s measles outbreak is behind the province, he said summer gatherings will continue to pose risks.
“What people need to understand is that measles does impact children, but it can also impact adolescents and adults as well, especially those who are unvaccinated,” Dr. Butt said.
“You have to break the chain of transmission by getting everyone vaccinated.”