Canada heading toward major measles outbreak without vaccine boost, new modelling suggests

March 16, 2024 Canuck

This story is part of CBC Health’s Second Opinion, a weekly analysis of health and medical science news emailed to subscribers on Saturday mornings. If you haven’t subscribed yet, you can do that by clicking here.

As measles cases keep appearing in more parts of the country, new projections suggest there’s a high chance Canada may experience a “sizable outbreak” — with anywhere from dozens to thousands of people infected if the disease strikes communities with low vaccination rates.

As of Friday, at least 31 cases of measles have been reported so far this year across Canada, according to a CBC News tally of provincial and regional figures released by public health teams.

That’s already the largest annual total since 2019 and more than double the number of cases reported last year, as medical experts fear the number will rise while more Canadians travel in and out of the country this month for March break.

New projections from a team at Simon Fraser University (SFU) in British Columbia show the grim possibilities. The modelling suggests that vaccine coverage of less than 85 per cent can lead to dozens of cases within small communities — or even hundreds if immunization rates are lower.

The SFU team shared the modelling exclusively with CBC News and also provided it to federal public health officials. CBC News also shared the findings with outside experts to review.

The modelling identified one example scenario: In a 1,000-person community with a 75 per cent vaccination level — and slower public health efforts to track and isolate cases — a measles outbreak could grow to a median size of 100 or so people, the team found.

That would mean roughly 20 hospitalizations, based on typical rates of severe disease from this highly contagious infection, which can lead to serious complications, such as pneumonia, brain inflammation and death.

The researchers also built projections for larger communities, and with stronger tracking and isolation efforts, and showed major outbreaks can occur even under those circumstances.

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In an 8,000-person community with 60 per cent vaccination coverage, for example, the team estimated there would be about 1,000 cases. If the vaccination rate was only 55 per cent, the outbreak could hit nearly 3,000 cases. (Larger outbreaks can also lead to deaths, since the death rate from a measles infection is about one to three out of every 1,000 cases.)

And some Canadian communities have uptake rates far lower than that, publicly available data shows.

“The worst-case scenario is that measles outbreaks aren’t contained,” warned researcher Caroline Colijn, an epidemiologist and mathematician at SFU in Burnaby, B.C., who helped prepare the study. “Canada could even lose its elimination status for measles, which it’s had since 1998.”

Montreal area an early hot spot

Dr. Brian Ward, a longtime measles researcher and professor at McGill University in Montreal, said he wasn’t surprised by the team’s projections, adding there’s “no question” Canada is headed toward major outbreaks.

“We will have, I would think, very large outbreaks — many hundreds of people — in the coming year or two, simply because there are so many people who are susceptible,” he said.

He described it as a potential “forest fire” of infections.

“If you have a huge amount of dead wood on the forest floor, it’s fuel. And in the case of a measles epidemic, susceptible individuals are the fuel,” Ward said.

“And if you have a ton of fuel in the bottom of a forest and then you throw a spark in, like an imported case … you can very rapidly have tens, then hundreds, then thousands of cases.”

Quebec is proving to be an early hot spot with 21 cases, most in the Montreal area, as the province races to stop local transmission through pop-up measles, mumps and rubella (MMR) vaccination clinics, launched earlier this week in multiple Montreal hospitals and schools.

WATCH | Measles may be spreading in some communities:

Measles may be spreading in some communities, health officials warn

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Measles cases in greater Montreal and north of Toronto are concerning health officials because two of them are not connected to international travel and are also not tied to any other known cases in Canada. They say this might mean the virus is spreading in the community, and they urge people to get vaccinated.

“The level of coverage of vaccination is not enough for us,” Dr. Luc Boileau, Quebec’s public health director, warned in an interview with CBC News.

He estimated there could be 100,000 children in the province’s school system who haven’t been fully vaccinated against measles — and that’s not including those in preschool and kindergarten who may be simply too young to have finished the standard two-dose regimen for the MMR vaccine.

While overall vaccination rates in Quebec remain high, Boileau stressed that there are some schools in the Montreal area where uptake for the measles vaccine is far lower, falling below 50, 40, or even 30 per cent.

“Those are places that are really at risk to have big outbreaks of measles,” he said.

Ward said susceptible communities have either been historically hesitant to get vaccinated, are currently refusing vaccines or are simply lagging behind on their shots.

In some cases, that could mean people playing catch-up after delays caused by the COVID-19 pandemic, or new immigrants who may be coming from parts of the world where children receive only one dose of the MMR shot, which doesn’t offer protection as robust as two rounds.

Caroline Colijn, an epidemiologist with Simon Fraser University, sits at a table at her Vancouver home. — ‘The worst-case scenario is that measles outbreaks aren’t contained,’ warns researcher Caroline Colijn, an epidemiologist with Simon Fraser University who helped prepare the study. ‘Canada could even lose its elimination status for measles, which it’s had since 1998.’ (Dillon Hodgin/CBC)

Pockets of low vaccination rates

Canada’s overall MMR vaccination rate is also falling, and it hit a low point mid-pandemic, the latest available data suggests.

In 2021, the year after the pandemic was declared, only 79 per cent of children had two doses by their seventh birthday — meaning two in 10 kids weren’t fully up-to-date on their shots. That’s down from 87 per cent just four years earlier.

What’s more important, SFU’s Colijn stressed, is the variability of that coverage within different communities.

In Alberta, for example, 75 per cent of all children up to the age of seven had both doses of the MMR vaccine in 2022, according to a provincial database. But the same data, broken down by region, shows vast disparities in uptake.

In many areas, including neighbourhoods in Calgary, the vaccination rate was above 80 per cent — while more than 100 communities fell below that level, including several with uptake below 50 per cent.

The County of Two Hills, a 3,000-person municipality in northeastern Alberta, had the province’s lowest MMR vaccination rate for children, at just 32 per cent.

“It’s not equal risk to all Canadians; it’s a really high risk in settings like that,” Colijn said.

A spokesperson for the Public Health Agency of Canada (PHAC) told CBC News that it is working closely with provinces and territories to monitor measles cases nationally.

The agency also agreed that modelling conducted by Canadian universities and PHAC’s own modellers suggests that “outbreaks are likely” if an infected individual brings an infection into schools or communities where vaccine coverage isn’t high enough.

“The lower the proportion of people vaccinated in these settings, the more likely an outbreak is to occur, and the larger it is expected to be,” PHAC spokesperson Mark Johnson wrote in an emailed statement.

Canadian cases follow massive outbreaks in Europe

While Canada hasn’t experienced an explosive outbreak yet this year, it’s happened before, including hundreds of cases reported in Quebec in 2011 and B.C. three years later.

Already, this year’s country-wide measles case tally isn’t far off the grand total in the United States — with 58 cases reported across 17 states as of March 14 — even though its population is nearly nine times the size.

That North American spike also follows massive outbreaks in the European region, with a 45-fold increase in cases across the area between 2022 and 2023. The region — which includes 53 member states in Europe and Central Asia — reported nearly 60,000 total infections last year, resulting in thousands of hospitalizations and 10 measles-related deaths, the World Health Organization announced in late February.

McGill University’s Ward said he expects the U.S. will eventually hit that level of infections, with Canada not far behind.

WATCH | Modelling suggests Canada could experience measles outbreaks:

Major measles outbreak expected without better vaccination rates, modelling shows

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Duration 2:40

Quebec has launched measles vaccination clinics as officials fear a major outbreak would put thousands of children at risk of getting very sick or even dying. The number of confirmed cases in Canada in 2024 has already exceeded the total for 2023, and modelling shows it could quickly get out of hand if vaccination rates don’t increase.

While pandemic disruptions may have curbed vaccine uptake, immunization rates didn’t trigger the return of vaccine-preventable diseases such as measles until more recently, given the public health interventions that were in place to combat COVID-19, said epidemiologist and researcher Nazeem Muhajarine of the University of Saskatchewan in Saskatoon.

“The concern is that immunization rates will not return to their pre-pandemic levels because of widespread anti-vaccine and anti-science activities in Canada and in the U.S, which really took off during the pandemic,” he said.

Muhajarine, who reviewed the new SFU modelling, said the projections make it clear that public health efforts to track and isolate cases are necessary — and the single biggest difference in outbreak size is a community’s vaccination rate.

That’s because measles is simply so contagious, he stressed, with each exposure leading to 15 or more other infections among people who haven’t been vaccinated or previously infected.

Outbreaks ‘inherently unpredictable’

Colijn’s modelling study does have multiple limitations, including the fact that researchers couldn’t know what the age distribution would be in an affected community — a factor that can skew rates of severe illness, since young children are less likely to be vaccinated and more likely to have serious complications.

Outbreak sizes are also “inherently unpredictable,” the researchers note.

Colijn stressed that models like the one from her team are meant to help Canada avoid dire consequences as more measles cases are imported from abroad, given the spike in global cases over the last year — even if they’re not a sure-fire prediction of what’s to come.

“Models are more like headlights than crystal balls,” she said. “If headlights show you something in your path and you can avoid it, you don’t say the headlights were wrong.”

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