A dose of the measles, mumps, and rubella (MMR) vaccination awaits the next patient during a vaccine clinic at Southwestern Public Health in St. Thomas, Ont., on March 4.Geoff Robins/The Canadian Press
The highly contagious measles virus continues to race through Ontario, with 155 new cases recorded over the past week, adding to an outbreak that has prompted New York state to issue a travel advisory urging residents to get vaccinated before crossing the northern border.
In its weekly update released Thursday, Public Health Ontario said there have been 816 measles cases diagnosed in the province since last October. Roughly one-fifth of the caseload has been reported over the past week, with 138 infections confirmed and 17 considered probable. Cases have been mostly concentrated in Southern Ontario and appeared in two additional public health units this week.
“The sharp increase in the number of outbreak cases and the geographic spread in recent weeks is due to continued exposures and transmission among individuals who have not been immunized,” the report states.
In depth: On the ground in the epicentre of Ontario’s measles outbreak
The province is experiencing the worst outbreak since measles was declared eliminated in Canada in 1988 after widespread vaccination. Measles activity has since largely been connected to travel, also referred to as “measles importation,” said the provincial agency. An increase in global measles activity last year is now rearing its head in Canada, particularly in Ontario and Manitoba.
The New York State Department of Health issued a travel advisory last week that highlighted Ontario’s outbreak and how easily the virus can jump from one person to another.
“Measles is only a car ride away!” reads the April 2 advisory. “Around 90 per cent of people who are exposed to a person with measles will become infected if they are not vaccinated. Because measles is so contagious, it easily crosses borders.”
The best form of protection is the measles-mumps-rubella vaccine, it said. One dose is about 93-per-cent effective at preventing measles; two doses, about 97-per-cent effective.
“Currently, measles outbreaks are happening in parts of the United States and Canada, especially in Ontario, and around the world. With spring and summer travel season approaching, anyone who is not protected can get measles while travelling and can easily spread it to others when they return home,” the advisory said.
Ontario chief medical officer stands by vaccine exemption law despite rising measles cases
Nearly 500 U.S. cases have been reported in the first three months of this year, surpassing the total number of measles infections in all of 2024. The majority of cases have been identified in Texas and New Mexico, primarily among children who have not received the vaccine. In Texas, two children have died.
U.S. Health and Human Services Secretary Robert F. Kennedy Jr., who has a history of sowing doubt about vaccines, recently endorsed the measles-mumps-rubella vaccine in a social media post. But in a separate post, he also praised doctors who used unproven treatments for measles.
There have been no deaths in Ontario, but 61 cases have required hospitalization. All but four of those patients were not immunized against measles, including 47 children.
Ninh Tran, the medical officer of health at Southwestern Public Health, serving a region south and east of London, which is bearing the brunt of the provincial outbreak, said in an update Thursday that 343 cases have been reported to the health unit since October. He spoke directly to parents and guardians in the region.
“If your child is already four and awaiting their second dose, don’t delay getting it, especially if you live, travel, play or worship in an area such as ours experiencing a measles outbreak,” he said. “If your child is younger than four and has an exposure during this outbreak, talk to your health care provider about options for a second dose.”
Vaccinations, across all ages, increased 130 per cent from Jan. 1 to April 8 over the same period last year, Dr. Tran said. That’s 940 additional doses in arms, an increase of 535 over last year.