TORONTO — Ontario is seeing a massive surge in measles cases as an ongoing outbreak spreads in the province among mostly unvaccinated people.
Public Health Ontario is reporting 372 total cases since an outbreak began on Oct. 28, 2024. That’s a jump of 195 cases since the agency’s last report on Feb. 27.
The public health agency described the latest numbers of the highly contagious airborne disease as being a “sharp increase,” attributing the outbreak expansion to transmission among unimmunized children and teens.
Dr. Christine Navarro, a public health physician at Public Health Ontario, says the agency is on alert and she expects there to be more cases in the coming weeks.
"It is unusual to see this kind of spread. It's not something that we've experienced in the province in many, many years, certainly not since elimination of measles in Canada in 1998," Navarro said.
Almost all of the new cases are connected to an interprovincial outbreak first reported in New Brunswick, which has also spread to Manitoba.
The spread has resulted in 31 hospitalizations in Ontario, including one child who required intensive care. Of those who were hospitalized, 30 were unvaccinated and one person’s immunization status was unknown.
Seven cases were reported in pregnant people — five unvaccinated, and two had two doses of the measles vaccine. Navarro said when people get infected despite being immunized, it's called a "breakthrough" case, which they do expect to occasionally see.
"We do expect there to be some proportion of cases among people who have had one dose and among people who have had two doses, but really, primarily the spread, it is among the those who are unimmunized," she said.
Breakthrough infections tend to be more mild because the vaccine reduces the risk of severe outcomes, she said. Those can include pneumonia, inflammation of the brain and death, and in pregnancy lead to spontaneous abortion, premature labour and infection of the newborn.
One newborn was infected with measles, the report shows, but Navarro said she couldn't speak to the infant's health status.
The measles-mumps-rubella vaccine is typically given between 12 to 15 months of age, though it can be given as early as six months in cases where there is higher risk of exposure. The booster — which also has protection against chickenpox — is given at four to six years old.
In the last school year, only 70 per cent of seven-year-olds were fully vaccinated against measles, Public Health Ontario data shows. The agency says this represents a “large decline” from rates before the COVID-19 pandemic, as seen with many of Ontario’s routine publicly funded immunization programs.
The National Advisory Committee on Immunization says 95 per cent is needed to ensure Canada stays free of endemic measles.
In Ontario, seven more public health units are reporting cases, bringing the total number to 11. Meanwhile, people are being warned about potential exposures in common public spaces that include restaurants, grocery stores, community centres and Toronto Pearson International Airport.
Most of the sick people are in the Southwestern Public Health region, where almost half of the cases are reported, and neighbouring Grand Erie, which has close to 27 per cent.
But measles has also travelled hundreds of kilometres away from that area, spanning west to Windsor and east to Niagara.
The number of cases reported in Ontario over the past five months is almost four times what the province saw in the decade between 2013 and 2023.
Symptoms of the highly contagious respiratory virus include fever, a red blotchy rash, red watery eyes and cough.
This report by The Canadian Press was first published on March 13, 2025.
Canadian Press health coverage receives support through a partnership with the Canadian Medical Association. CP is solely responsible for this content.
Hannah Alberga, The Canadian Press
