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Ontario is in the midst of its worst outbreak of measles in more than 20 years, according to the latest report from Public Health Ontario.

As of February 26, 2025, there have been 177 cases, an increase of 78 since the previous summary on February 13, and far surpassing the 101 cases in total that were recorded in Ontario between 2013 and 2023.

The outbreaks are centered in the southwestern part of the province and occurred when several people from Ontario were exposed to the measles virus while in New Brunswick. All cases have been in people born in 1970 or after; 141 cases have been recorded in children and adolescents, and 36 cases in adults. The vast majority (98%) were unvaccinated. So far, 18 people have been hospitalized, including one child who required intensive care.

Before the pandemic, the annual number of measles cases in Ontario ranged between 7 and 22.

“We would have very low numbers, and those would all be importation-related, due to people coming into Canada after visiting a country where measles is endemic,” Christine Navarro, MD, a public health physician with Public Health Ontario, Toronto, told Medscape Medical News. The current numbers “represent a significant increase from what we would typically see during any given year.”

Vaccination. Vaccination. It’s That Simple

The surge is cause for concern, experts agreed.

“Measles is a very dangerous disease,” Dawn Bowdish, PhD, associate professor at McMaster University, Hamilton, Ontario, told Medscape Medical News. “Between 1 in 3 and 1 in 5 kids need hospitalization or serious medical care. It leads to birth defects in pregnant women and lost pregnancies, premature births, and all sorts of problems with mom and baby.”

“Not only does measles get into your lungs, it also kills your immune cells, and, as a result, people who get measles tend to have a lot more infections and require more antibiotics until their immune memories come back,” she warned.

Furthermore, measles is “a very transmissible infection,” Isaac Bogoch, MD, infectious disease specialist at Toronto General Hospital, Toronto, told Medscape Medical News. “There has been a large global resurgence of measles, and we live in an era of unprecedented human mobility, so of course we will inevitably import a case of measles into Canada once in a while.”

When a case of measles is introduced into Canada, he said, “we should have population-level protection through immunity, and that means we need our vaccine rates hovering at around 95%. When our vaccination rates are 95%, you won’t see subsequent transmission or spread. And if we look at who is getting infected with measles, it’s almost exclusively people who are unvaccinated, and it’s really unvaccinated children. It’s that simple.”

Overall, Canada is doing well, he said. Vaccination rates are generally high, but there are small areas where rates are very low. When measles is introduced into these areas, it will inevitably spread as it has in Ontario.

Reassurance Is Key

Measles was eliminated in Canada in 1998, thanks to 90%-95% vaccination coverage and a low level of transmission for at least 1 year. But now, fewer parents are opting to get their children immunized, in some cases due to disinformation about the vaccine’s safety.

“We need a high percentage of people to be immunized to attain herd immunity. If there are susceptible people, mainly those who are not immunized, then the virus will continue to spread like a fire that only burns itself out when there is no more fuel,” explained Anna Banerji, MD, MPH, a pediatric infectious disease specialist at the University of Toronto, Toronto, and founder of the North American Refugee Health Conference.

It’s important that doctors discuss any vaccine hesitancy issues their patients might have and reassure them that the measles vaccine is safe and effective, she told Medscape Medical News.

Measles is a highly infectious virus that can result in serious consequences, she added. “It can cause pneumonia, deafness, blindness, it can go to the brain and cause encephalitis. I tell them the vaccine was licensed in 1963 and is one of the oldest vaccines. It is highly effective and causes a 99% reduction in cases. Before the vaccine, almost everyone got the measles.”

Banerji also suggested sharing Roald Dahl’s letter about losing his daughter to measles in 1962, noting that “many parents have read his books.”

Bowdish agreed that it is paramount to get good quality information to the general public to combat vaccine hesitancy. “I think part of the problem is that many people don’t remember measles, so they don’t have a sense of just how dangerous it is,” she said. “It’s like a collective forgetting. The challenge now is that there are pockets of people who are not vaccinated, and that is the reason for these outbreaks.”

The outbreaks are not restricted to measles, Bogoch added. “What we are seeing is a resurgence of vaccine-preventable illnesses. A few months ago, we were seeing a large whooping cough outbreak. Now it’s measles.”