Here is what you need to know about the spike in measles cases in Ontario

A measles vaccine is shown on a countertop at the Tamalpais Pediatrics clinic Friday, Feb. 6, 2015, in Greenbrae, Calif.

Ontario has seen an alarming spike in the number of measles cases, hitting a more than 10-year high. Here is what you need to know about how the virus is spreading in the province.

How many cases have there been?

Measles cases in Ontario have reached their highest level in more than a decade, with 350 cases reported since October 18, 2024, when a travel-related case in New Brunswick spread to Ontario and Manitoba, leading to outbreaks in those provinces.

In 2025 alone, there have been 252 confirmed and 66 probable cases of measles. All but five cases have been linked to the multi-jurisdictional outbreak and those five cases all had a history of travel, according to the latest surveillance report by Public Health Ontario which included data up to March 12, 2025.

Since Feb. 27, Ontario has seen the number of cases jump by 173, including 131 confirmed and 42 probable cases.

PHO said that 74 per cent of the outbreak cases were in children and adolescents while 24.9 per cent were in adults. Seven outbreak cases were in pregnant individuals, including five that were unimmunized and two that had previously received two doses of the measles vaccine. One outbreak case was in a newborn, PHO said.

Why are measles cases spreading?

According to the report, the number of outbreak cases and the geographic spread in recent weeks is “attributable to ongoing exposures and transmission among individuals who are unimmunized.”

Infectious diseases specialist Dr. Isaac Bogoch told CP24.com Friday that the virus has a “knack” for finding under-vaccinated communities.

“Most Canadians are immune but that’s not good enough because this one is extremely transmissible,” he said.

He noted that a community needs a vaccine rate of about 95 per cent to prevent transmission.

“There are pockets in the country that are unvaccinated and this virus will exploit those pockets,” he said.

Where are we seeing cases?

Public Health Ontario (PHO) says the outbreak is impacting 11 public health units across the province. In a statement released Friday, Dr. Kieran Moore said the majority of Ontario’s cases are concentrated in southwestern Ontario and are among unvaccinated people.

An estimated 181 cases, or roughly 51 per cent of reported cases, were found in the Southwestern Public Health Unit, which includes Oxford County, Elgin County, and the City of St. Thomas.

Are there cases in Toronto?

No outbreak cases have been reported by Toronto Public Health and so far this year, there have been only two confirmed cases of measles. But Bogoch said exposures can happen in the region at any time.

“Let’s not pretend that there is some forcefield around Toronto,” Bogoch said. “Certainly there could be pockets in the GTA that are under-vaccinated.”

He noted that in the GTA, there are ample opportunities for spread, including at malls, on transit, and at the country’s largest airport.

“Where there are under-vaccinated pockets, unfortunately there are more opportunities for the virus to find them and exploit those,” he added.

How many cases has Ontario seen in previous years?

According to PHO, between 2013 and 2023, there were a total of 101 confirmed cases of measles reported in the province, less than half of the number reported in the first three months of this year. Between the years of 2013 and 2019, the annual number of measles cases in Ontario ranged between seven and 22, according to PHO data. During the COVID-19 pandemic between 2020 and 2022, one case was reported. Seven cases were reported in 2023.

In 2024, that number surged to 64, a total that included 37 cases that were linked to the multi-jurisdictional outbreak.

How can we slow down transmission?

Experts say the most effective way to limit the spread is by getting vaccinated.

“The measles, mumps and rubella (MMR) vaccine has been in use for more than 50 years and is proven to be one of the safest and most effective vaccines available,” Moore continued.

“All Ontarians should ensure themselves, and their children, are up to date on their vaccinations. Children who are fully immunized with two doses of the measles vaccine are nearly 100 per cent protected, with one dose estimated to be up to 95 per cent protective.”

Children receive two doses of the vaccine at 12 months and between four and six years old as part of routine childhood immunizations.