A play-by-play of how measles outbreaks can spiral out of control

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Measles infections are ticking up and up across multiple provinces, with more cases already this year than all of 2024.

Local public health alerts of new cases come with detailed descriptions of the person’s whereabouts before they were diagnosed.

We’re told which hours and minutes a passenger spent in airports in Vancouver, Toronto, and Fredericton, how long other patients were in an emergency department north of Toronto or family health clinic in eastern Ontario, and just how long someone visited a sit-down chain restaurant in Quebec’s Laurentians.

The announcements raise a couple questions: Why do measles cases seem to be slipping through health-care professionals’ fingers — or even going unrecognized by patients or their families?

And, why give such detailed information on where a person went?

The answers lie in how incredibly contagious the measles virus is, coupled with the cornerstone role vaccination plays in stopping spread.

“What those announcements are trying to do is really make people aware so that they can protect … others around them and watch out for their own symptoms,” said infectious disease expert Caroline Colijn, a professor of mathematics who holds a Canada 150 Research Chair at Simon Fraser University in Burnaby, B.C.

Last month, Colijn and her co-authors published a modelling study on measles to see what happens when the respiratory illness is introduced to a community, and what level of vaccination coverage or previous immunity from infection is needed to prevent outbreaks.

“Because measles is so incredibly infectious, it can spread rapidly, even if most people in a community, 80 per cent, for example, are immunized,” Colijn said. “The usual threshold is that you would want 95 per cent of the people in a community to either have been vaccinated or to have had previous measles exposure.”

However, vaccination rates have been falling in Canada and elsewhere, and the early symptoms of measles may go unrecognized — adding to the risk of spread.

Hardy and highly contagious virus

Measles spreads through the air when someone coughs, sneezes or talks, so even spending a few minutes in that same airspace can pose an infection risk to someone who isn’t vaccinated, such as an infant who is too young to receive the shots. A single infected person can infect 90 per cent of their close contacts, if they’re unvaccinated. 

The virus that causes measles is also hardy, lingering on surfaces for two hours after an infected individual leaves.

That’s why case counts can spiral quickly. A single person, on average, infects 12 to 18 others who are susceptible to measles.

That means if one patient infects 15 others, who each infects 15 others, you end up with 225 cases. If each of those people infects 15 others, you’re at 3,375 cases, and so on. 

Another wrinkle is that the symptoms of measles overlap with more common respiratory illnesses, including influenza, which is also circulating in Canada now.

When someone who is vulnerable to the measles virus is exposed, they may show the “3 C” landmarks of the infection about 10 days later:

• Cough.

• Coryza, or inflammation of nasal mucous membranes, which leads to a runny nose.

• Conjunctivitis or redness of the eyes.

“These are very similar to things we would see with the flu,” said Mark Kirchhof, president-elect of the Canadian Dermatology Association and an associate professor of dermatology at the University of Ottawa. “It can be easily confused.”

When people unknowingly infected with measles chalk up the symptoms to influenza, a cold or COVID-19, especially early on, they may interact with family, go to school, work and other public places and expose others who haven’t been vaccinated, Kirchhof said.

That’s why local public health officials and physicians encourage people to call ahead before coming to a clinic if the person is unvaccinated and showing cold symptoms. That way, staff can cordon off an area to prevent further spread of measles.

Measles rash appears on the face first

When the Canadian Dermatology Association observed an uptick in measles in 2023, the group started raising awareness about the measles rash, other symptoms and who is vulnerable to complications.

Kirchhof said dermatologists can lend a hand at helping colleagues to recognize measles, which many doctors have only read about in textbooks and never seen in real life.

Two or three days after the symptoms of cough, a runny nose and conjunctivitis arise in someone infected with measles, a cluster of white dots will often appear on the inside of the cheeks near the molars. These are known as Koplik spots after the pediatrician who first identified the pattern’s link to measles infection.

WATCH | Mapping the spread of measles in Canada in 2025: 

Measles was declared eradicated in Canada in 1998, but with vaccination rates dropping, new cases are surging; 95 people have been infected in just the first two months of this year, compared to 147 for all of 2024.

Then, a few days after the Koplik spots appear, a characteristic rash starts on the face and spreads downwards to the neck, legs, arms and feet.

“The rash itself is red dots,” Kirchhof said. “Some of them you can feel, and some of them might be flat. Sometimes they can cluster together to form larger red areas over the body, and in some patients it can be somewhat itchy.”

Depending on skin tone, the brightness of the rash may vary, doctors say.

It’s the delay between exposure and when measles symptoms erupt, followed by the time lag between types of symptoms that explains why someone who eventually seeks treatment may pass through airports, schools, grocery stores and hospitals before being diagnosed.

Initially, measles affects the respiratory tract, and common complications include ear infections.

But measles can also lead to high hospitalization rates. It is estimated nearly one to three of every 1,000 children who become infected will die from complications, such as pneumonia and a swelling of the brain called encephalitis.

There is no specific treatment for measles infection, which needs to run its course.

Countering fall in childhood immunization rates

The best prevention, public health officials in Canada and internationally stress, is vaccination. 

But, amid declining vaccination rates, some people on the internet or on social media may promote treatments that lack evidence, Kirchhof said, such as vitamins or supplements that maintain general health, though don’t specifically treat measles.

In contrast, federal figures reflect the power of high vaccination rates to curb outbreaks. Before measles, mumps and rubella (MMR) vaccinations started to roll out in Canada in the 1970s, tens of thousands of cases occurred each year.

More recently, falling vaccination rates in children in Canada and international travel reintroducing the virus have also contributed to outbreaks, scientists say.

Sarah Khan, an infectious diseases specialist and associate professor at McMaster University, was part of the team who cared for an unvaccinated child who died of measles last year in Hamilton.

“It’s devastating when you have to manage a vaccine-preventable disease that results in a fatality,” Khan said.

Given the timing and other factors, Khan said managing even a single case of measles can be a challenge.

Doctors say those at highest risk of exposure, such as an infant or someone who is pregnant, may benefit from a medicine called immune globulin to reduce the risk of developing measles.

“It really does require early recognition, early notification and a lot of coordination for all of the exposed individuals,” Khan said.