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

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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.

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."

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.